monitor: Removed unused variable

Fix clang-tidy error:
```
../monitor/fan.hpp:277:12: error: private field '_numFailedSensor' is not used [-Werror,-Wunused-private-field]
```

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I812c417d6c84adcf6e9045299e4705e8a58a4089
diff --git a/monitor/fan.hpp b/monitor/fan.hpp
index 5892ba2..72f4eab 100644
--- a/monitor/fan.hpp
+++ b/monitor/fan.hpp
@@ -272,11 +272,6 @@
     const size_t _numSensorFailsForNonFunc;
 
     /**
-     * The number of failed sensors
-     */
-    size_t _numFailedSensor = 0;
-
-    /**
      * @brief The current functional state of the fan
      */
     bool _functional = true;