sched: Remove the obsolete exit_state/signal hacks
account_group_xxx() functions check ->exit_state to ensure that
current->signal is valid and can't go away. This is not needed
since ea6d290c, task->signal is pinned to task_struct.
The comment and another hack in account_group_exec_runtime() refers
to task_rq_unlock_wait() which was already removed by b7b8ff63.
Signed-off-by: Oleg Nesterov <firstname.lastname@example.org>
Signed-off-by: Peter Zijlstra <email@example.com>
Signed-off-by: Ingo Molnar <firstname.lastname@example.org>