io_uring: remove io_register_submitter

this is no longer needed, as submitter_task is set at creation time.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Fixes: 97bbdc06 ("io_uring: add IORING_SETUP_SINGLE_ISSUER")
Signed-off-by: Jens Axboe <axboe@kernel.dk>