Revert "Smack: Handle io_uring kernel thread privileges"

This reverts commit 942cb357.

The io_uring PF_IO_WORKER threads no longer have PF_KTHREAD set, so no
need to special case them for credential checks.

Cc: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>