I no longer meet the requirements to deposit into my Roth IRA. There's not a huge amount of money in it. Rather than have this relatively small account laying around, requiring periodic monitoring and maintenance, I'd rather roll it onto my traditional IRA. Is this possible?
No, you cannot convert or roll over a Roth IRA directly to a traditional IRA. At best, you could withdraw funds from the Roth IRA and contribute them to a traditional IRA. This is mostly pointless because: (1) You will owe taxes and penalties on the Roth earnings if you're under 59.5, which you won't owe if you keep the money there until retirement. (2) If you are eligible to make deductible contributions to a traditional IRA and would normally fully fund it, the money from the Roth IRA will simply displace other contributions you would have made. (3) If you are not eligible for deductible contributions, you are just turning the money from non-taxable to taxable in retirement.
The Roth IRA has unique advantages; rather than trying to get rid of it, consider trying to grow it. If you're not eligible to contribute directly, you could designate some or all of your current traditional IRA contributions as non-deductible and then convert them to a Roth IRA. This is the "backdoor Roth". You can also convert deductible contributions, and earnings, if you pay the taxes (and this way you get something for the taxes, because the further growth won't be taxed). Note, you have to prorate your conversion based on your overall traditional IRA holdings, so can't purely convert after-tax (non-deductible) contributions, unless you do a reverse rollover to a 401(k) to get the pre-tax money out of the traditional IRA first.
By locking in today's tax rate, a Roth provides tax diversification in case tax rates are higher in the future. The monitoring overhead could be minimized by keeping your traditional and Roth IRAs at the same institution (via an ordinary custodian transfer) if they aren't already.
If you are determined to get rid of the Roth IRA, the least damage would be to withdraw it, pay the taxes and penalties, and then invest in low-cost tax-efficient index funds in a taxable account.
Rather than have this relatively small account laying around, requiring periodic monitoring and maintenance, I'd rather roll it onto my traditional IRA. Is this possible?
Another option is to do a transfer to another trustee. The money will still be a Roth IRA, it will just be managed by a different trustee. A trustee should be able to handle multiple different investment types. They should be able to have Roth IRA, Traditional IRA, and taxable investments in the same account. My wife's account with one trustee by coincidence also has her 401K money.
The idea would be for you to move the funds to the same trustee that has your Traditional IRA. That will result in the Roth IRA and the Traditional IRA can be checked with one single login.
The best part of the trustee-to-trustee transfer is that it has no tax implications, and the new trustee can lead you through the process. You have to decide if you want to end up with the trustee that currently has the Roth IRA or the one that has the Traditional IRA to be holder of both types. The procedure to do the transfer of the accounts will be essentially the same, you just have to decide where you want the funds to go.
As others have explained you can still use the "backdoor Roth" to turn contributions to a traditional IRA into Roth contributions.
If you already have an established Roth IRA into which you contributed money in earlier years when your income was lower, there is no need to roll that money (or any of the earnings/gains on those contributions for that matter) into a Traditional IRA. You are entitled to have and to hold that Roth IRA for as long a you like. Now, if you put money into your existing Roth IRA for 2019 and now have discovered that for 2019 you are not eligible to contribute to a Roth IRA, then that money (and all earnings on it) needs to be recharacterized as a contribution to a Traditional IRA. Talk to a service representative of your IRA custodian and tell them that you are not eligible to contribute to a Roth IRA for 2019, that you have already made a contribution to the Roth IRA and now you want to recharacterize that contribution (and all earnings thereon) as a contribution to a Traditional IRA. They will tell you which forms you need to fill out, and transfer the money into your Traditional IRA account (establishing one for you if you don't already have a Traditional IRA account.
Be aware that anyone with taxable compensation can contribute to a Traditional IRA, but not everyone can deduct the contribution from taxable income. If your income is too high to contribute to your Roth IRA, it might also be high enough that you cannot deduct the contribution. Be sure to fill out Form 8606 to report to the IRS that you made a nondeductible contribution to your Traditional IRA. That way, the money will to be taxed again when you withdraw it in later years.