system: revert 657061bf behaviour with 'tick' timers

if tick object exists, repeat flag was set for the os_timer_arm()
do not fire more than once when repeating was not requested

code/espurna/system.cpp

@@ -399,6 +399,9 @@ void SystemTimer::callback() {
         return;
     }
 
+    if (_tick) {
+        os_timer_disarm(_armed);
+    }
     reset();
 }