sensor-mon: Remove unused variable

Fix clang-tidy error:
```
../sensor-monitor/threshold_alarm_logger.hpp:211:25: error: private field 'event' is not used [-Werror,-Wunused-private-field]
```

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I35ef3a74be5e1db36d39c6ffafbd11e5a784ba87
diff --git a/sensor-monitor/main.cpp b/sensor-monitor/main.cpp
index 1974065..6373b17 100644
--- a/sensor-monitor/main.cpp
+++ b/sensor-monitor/main.cpp
@@ -38,7 +38,7 @@
 
     ShutdownAlarmMonitor shutdownMonitor{bus, event, powerState};
 
-    ThresholdAlarmLogger logger{bus, event, powerState};
+    ThresholdAlarmLogger logger{bus, powerState};
 
     return event.loop();
 }