How can I remove this bathtub drain
The drain will unscrew... you might need a drain key (or expanding tub drain remover). Here's a video: https://www.youtube.com/watch?v=T_0vYme1Q88



Or you might be able to grab it with a pair of channel locks:



But if you don't care about it any more then you could use a dremel tool to cut a notch in it so that you can use a flat head screwdriver and a hammer to unscrew it.