Anyone know how to remove a bathtub drain stopper?

which kind is it? most just unscrew. Lift-and-turn or push-pull: open it, hold the base and turn the cap anticlockwise. If it spins without coming out, look for a small set screw under the cap. Toe-touch ones unscrew the same way. If you've got a trip lever on the overflow plate there's no real stopper in the drain at all - the plunger sits behind that plate. Undo the two screws on it and pull the whole linkage out.