Since the water heater had been drained earlier, you have pure anti-freeze in it. For all intents I'd leave it in and just do some flushing next spring. The antifreeze won't hurt the tank, but don't forget to flush it out before firing it up next spring.
If it bothers you, drain it and then buy one of these. Water Heater Tank Rinser - Camco 11691 - Water Heaters - Camping World
This will help you get the antifreeze flushed out without messing up your existing winterize job on the rest of the plumbing. Using one of these rods annually at the end of the season also flushes out any mineral deposits and crud within the tank. I had one of these that I used annually on my sob trailer. I owned that trailer for 14 years and that original tank was still doing its job on the day I sold it.
Also note that tanks never drain completely anyway. What little water is left that freezes will not damage the tank since it really has lots of room for expansion when it turns to ice. I tend to leave the drain plug out and place it on the little ledge where the door hinge is located. Just remember to put it back in next spring!