io-wq: remove spurious bit clear on task_work addition

There's a small race here where the task_work could finish and drop
the worker itself, so that by the time that task_work_add() returns
with a successful addition we've already put the worker.

The worker callbacks clear this bit themselves, so we don't actually
need to manually clear it in the caller. Get rid of it.

Reported-by:  <syzbot+b60c982cb0efc5e05a47@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>