io-wq: remove header files not needed anymore

mm related header files are not needed for io-wq module.
remove them for a small clean-up.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>