io_uring: don't take ctx refs in task_work handler

__tctx_task_work() guarantees that ctx won't be killed while running
task_works, so we can remove now unnecessary ctx pinning for internally
armed polling.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>