block/linux-aio: Drop unused BlockAIOCB submission method

Callback-based laio_submit() and laio_cancel() were left after
rewriting Linux AIO backend to coroutines in hope that they would be
used in other code that could bypass coroutines. They can be safely
removed because they have not been used since that time.

Signed-off-by: Julia Suvorova <jusual@mail.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>