How can I remove this bathtub drain?
You need to buy something to break up the rust fusion you have going on.  WD-40 makes a variety but there are many at your local DIY center.

I would then tap around the drain.  Then get a blunt/dull metal block or chisel and hold it against the inner straps and tap on those counter-clockwise.  The tapping (with a hammer or mallet) is probably the key to breaking up the rust bond.  You can't just start cracking it hard or you could snap one of the center straps or warp the outside.