remove background start timestamp reset on sleep

src/heartratetask/HeartRateTask.cpp

@@ -37,7 +37,6 @@ void HeartRateTask::Work() {
             state = States::Idle;
           } else if (state == States::Measuring) {
             state = States::BackgroundWaiting;
-            backgroundMeasurementWaitingStart = xTaskGetTickCount();
             StopMeasurement();
           }
           break;

src/heartratetask/HeartRateTask.h

@@ -42,7 +42,7 @@ namespace Pinetime {
       Controllers::HeartRateController& controller;
       Controllers::Ppg ppg;
       int lastBpm = 0;
-      TickType_t backgroundMeasurementWaitingStart;
+      TickType_t backgroundMeasurementWaitingStart = 0;
     };
 
   }