Simplify, drop SYNC_SHUTDOWN state file, not needed anymore

Similar to 6224166 (previous commit), this old code is a remnant of a
bygone era and not needed anymore.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/service.c

@@ -1422,7 +1422,7 @@ void service_monitor(pid_t lost, int status)
 {
 	svc_t *svc;
 
-	if (fexist(SYNC_SHUTDOWN) || lost <= 1)
+	if (lost <= 1)
 		return;
 
 	svc = svc_find_by_pid(lost);