Copter: remove unused error code and add comments

ArduCopter/defines.h

@@ -339,7 +339,7 @@ enum LoggingParameters {
 #define ERROR_SUBSYSTEM_MAIN                1
 #define ERROR_SUBSYSTEM_RADIO               2
 #define ERROR_SUBSYSTEM_COMPASS             3
-#define ERROR_SUBSYSTEM_OPTFLOW             4
+#define ERROR_SUBSYSTEM_OPTFLOW             4   // not used
 #define ERROR_SUBSYSTEM_FAILSAFE_RADIO      5
 #define ERROR_SUBSYSTEM_FAILSAFE_BATT       6
 #define ERROR_SUBSYSTEM_FAILSAFE_GPS        7   // not used
@@ -349,7 +349,7 @@ enum LoggingParameters {
 #define ERROR_SUBSYSTEM_GPS                 11
 #define ERROR_SUBSYSTEM_CRASH_CHECK         12
 #define ERROR_SUBSYSTEM_FLIP                13
-#define ERROR_SUBSYSTEM_AUTOTUNE            14
+#define ERROR_SUBSYSTEM_AUTOTUNE            14  // not used
 #define ERROR_SUBSYSTEM_PARACHUTE           15
 #define ERROR_SUBSYSTEM_EKFCHECK            16
 #define ERROR_SUBSYSTEM_FAILSAFE_EKFINAV    17
@@ -370,8 +370,6 @@ enum LoggingParameters {
 // subsystem specific error codes -- failsafe_thr, batt, gps
 #define ERROR_CODE_FAILSAFE_RESOLVED        0
 #define ERROR_CODE_FAILSAFE_OCCURRED        1
-// subsystem specific error codes -- compass
-#define ERROR_CODE_COMPASS_FAILED_TO_READ   2
 // subsystem specific error codes -- main
 #define ERROR_CODE_MAIN_INS_DELAY           1
 // subsystem specific error codes -- crash checker