io_uring: don't unnecessarily clear F_LINK_TIMEOUT

If a request had REQ_F_LINK_TIMEOUT it would've been cleared in
__io_kill_linked_timeout() by the time of __io_fail_links(), so no need
to care about it.

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