system: revert 657061bf completely

disarm is *required* by the underlying API, don't miss it on start()
while the func uses a resource lock internally, it does not seem to be the
source of system lock-ups encountered so far

code/espurna/system.cpp

@@ -399,10 +399,7 @@ void SystemTimer::callback() {
         return;
     }
 
-    if (_tick) {
-        os_timer_disarm(_armed);
-    }
-    reset();
+    stop();
 }
 
 void SystemTimer::schedule_once(Duration duration, Callback callback) {