My potatoes didn't do too great in the raised beds last season, because we dealt with a long drought. I must have left one behind as well, because one survived winter here as well. I'm leaving it alone just to see what happens. I attribute its survival to a much warmer and dry winter. If we had typical weather I believe they would have rotted from the moisture.

If you leave the potatoes, they will compete with the garlic for nutrients and block out light for the garlic. Plus, if the potatoes are ready before the garlic, you'll have to pull the garlic early to harvest the potatoes. If you have enough of each planted, you could also do nothing and just see what happens.

Here is a link that might be useful: My gardens