sched/core: ipipe: do not panic on failed migration to the head stage

__ipipe_migrate_head() should not BUG() unconditionally when failing
to schedule out a thread, but rather let the real-time core handle the
situation a bit more gracefully.

kernel/sched/core.c

@@ -5819,8 +5819,6 @@ int __ipipe_migrate_head(void)
 	if (likely(__schedule(false)))
 		return 0;
 
-	BUG_ON(!signal_pending(p));
-
 	preempt_enable();
 	return -ERESTARTSYS;
 }