pidfd: exit: kill the no longer used thread_group_exited()

It was used by pidfd_poll() but now it has no callers.

If it finally finds a modular user we can revert this change, but note
that the comment above this helper and the changelog in 38fd525a4c61
("exit: Factor thread_group_exited out of pidfd_poll") are not accurate,
thread_group_exited() won't return true if all other threads have passed
exit_notify() and are zombies, it returns true only when all other threads
are completely gone. Not to mention that it can only work if the task
identified by @pid is a thread-group leader.

Signed-off-by: Oleg Nesterov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Tycho Andersen <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

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