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 121b4fb1362cb705283ae6332c7e6a1b2d6e6d54.

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>