tomoyo: don't special case PF_IO_WORKER for PF_KTHREAD

Since commit 3bfe6106 ("io-wq: fork worker threads from original
task") stopped using PF_KTHREAD flag for the io_uring PF_IO_WORKER threads,
tomoyo_kernel_service() no longer needs to check PF_IO_WORKER flag.

(This is a 5.12+ patch. Please don't send to stable kernels.)

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>