io_uring: remove opcode check on ltimeout kill

__io_kill_linked_timeout() already checks for REQ_F_LTIMEOUT_ACTIVE and
it's set only for linked timeouts. No need to verify next request's
opcode.

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