Done....thanks for everyone's suggestions. I took a sledge hammer to
it. Since I had the missing corner piece, the concrete started to
separate nicely soley focussing on that area first. I did kind of
starting hitting at the slab from straight above, which ended up going
straight through the rotten subfloor below, so I modified my method and
started swinging more at an angle toward the areas that I had already
removed and it worked much better. Probably took me an 1 hour and half
to remove a 5'x7'x3" slab. Joist look good, subfloor has got to go
around the tub.

Thanks again for the suggestions.