Remove execution domain support

All users of exec_domain are gone, now we can get rid
of that abandoned feature.
To not break existing userspace we keep a dummy
/proc/execdomains file which will always contain
"0-0     Linux                   [kernel]".

Signed-off-by: Richard Weinberger <richard@nod.at>

kernel/exit.c

@@ -756,8 +756,6 @@ void do_exit(long code)
 
 	cgroup_exit(tsk);
 
-	module_put(task_thread_info(tsk)->exec_domain->module);
-
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
 	 */