Rover: remove unused channel_aux member

APMrover2/Rover.cpp

@@ -26,7 +26,6 @@ Rover::Rover(void) :
     param_loader(var_info),
     channel_steer(nullptr),
     channel_throttle(nullptr),
-    channel_aux(nullptr),
     channel_lateral(nullptr),
     logger{g.log_bitmask},
     modes(&g.mode1),

APMrover2/Rover.h

@@ -165,7 +165,6 @@ private:
     // primary control channels
     RC_Channel *channel_steer;
     RC_Channel *channel_throttle;
-    RC_Channel *channel_aux;
     RC_Channel *channel_lateral;
 
     AP_Logger logger;