io_uring: refactor io_async_cancel()

Remove extra tctx==NULL checks that are already done by
io_async_cancel_one().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/70c2a8b958d942e86958a28af0452966ce1095b0.1617287883.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>