io_uring: io_sq_thread() doesn't need to flush signals

io_uring: io_sq_thread() doesn't need to flush signals

We're not handling signals by default in kernel threads, and we never
use TWA_SIGNAL for the SQPOLL thread internally. Hence we can never
have a signal pending, and we don't need to check for it (nor flush it).

Signed-off-by: Jens Axboe <axboe@kernel.dk>