Remove a stale reference to the stop_scheduler_on_panic tunable, which

itself was removed in r243515.

MFC after:	1 week

sys/kern/kern_shutdown.c

@@ -700,10 +700,8 @@ vpanic(const char *fmt, va_list ap)
 	}
 
 	/*
-	 * We set stop_scheduler here and not in the block above,
-	 * because we want to ensure that if panic has been called and
-	 * stop_scheduler_on_panic is true, then stop_scheduler will
-	 * always be set.  Even if panic has been entered from kdb.
+	 * Ensure that the scheduler is stopped while panicking, even if panic
+	 * has been entered from kdb.
 	 */
 	td->td_stopsched = 1;
 #endif