io_uring/waitid: always remove waitid entry for cancel all

We know the request is either being removed, or already in the process of
being removed through task_work, so we can delete it from our waitid list
upfront. This is important for remove all conditions, as we otherwise
will find it multiple times and prevent cancelation progress.

Remove the dead check in cancelation as well for the hash_node being
empty or not. We already have a waitid reference check for ownership,
so we don't need to check the list too.

Cc: stable@vger.kernel.org
Fixes: f31ecf67 ("io_uring: add IORING_OP_WAITID support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>