ArduCopter: remove Log_Write_Performance which is not called anywhere

ArduCopter/Copter.h

@@ -782,7 +782,6 @@ private:
 
     // Log.cpp
     void Log_Write_Control_Tuning();
-    void Log_Write_Performance();
     void Log_Write_Attitude();
     void Log_Write_EKF_POS();
     void Log_Write_MotBatt();

ArduCopter/Log.cpp

@@ -631,7 +631,6 @@ void Copter::log_init(void)
 #else // LOGGING_ENABLED
 
 void Copter::Log_Write_Control_Tuning() {}
-void Copter::Log_Write_Performance() {}
 void Copter::Log_Write_Attitude(void) {}
 void Copter::Log_Write_EKF_POS() {}
 void Copter::Log_Write_MotBatt() {}