Revert "kernel: treat PF_IO_WORKER like PF_KTHREAD for ptrace/signals"

Revert "kernel: treat PF_IO_WORKER like PF_KTHREAD for ptrace/signals"

This reverts commit adf6461ce36573350744e3be4a3124475ea70de2.

The IO threads do allow signals now, including SIGSTOP, and we can allow
ptrace attach. Attaching won't reveal anything interesting for the IO
threads, but it will allow eg gdb to attach to a task with io_urings
and IO threads without complaining. And once attached, it will allow
the usual introspection into regular threads.

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