Remove bogus BUG() in kernel/exit.c

It's old sanity checking that may have been useful for debugging, but
is just bogus these days.

Noticed by Mattia Belletti.
diff --git a/kernel/exit.c b/kernel/exit.c
index 39d35935..93851bc 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -517,8 +517,6 @@
 	 */
 	BUG_ON(p == reaper || reaper->exit_state >= EXIT_ZOMBIE);
 	p->real_parent = reaper;
-	if (p->parent == p->real_parent)
-		BUG();
 }
 
 static inline void reparent_thread(task_t *p, task_t *father, int traced)