amiflop: clean up on errors during setup

The error handling in fd_probe_drives() doesn't clean up at all. Fix it
up in preparation for converting to blk-mq. While we're here, get rid of
the commented out amiga_floppy_remove().

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>