Tracker: process mavlink msgs from vehicle once

previously heartbeat messages from the vehicle could be processed twice.
Once at the top of hte handleMessage function where they were forwarded
onto the GCS and then again lower down in the function where all
received heartbeats were sent to the vehicle.

AntennaTracker/GCS_Mavlink.pde

@@ -434,10 +434,7 @@ GCS_MAVLINK::data_stream_send(void)
 
 void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
 {
-    if (g.proxy_mode == true)
-    {
-        if (chan == proxy_vehicle.chan)
-        {
+    if (g.proxy_mode == true && chan == proxy_vehicle.chan) {
             // From the remote vehicle.
             // All messages from the remote are proxied to GCS
             // We also eavesdrop on MAVLINK_MSG_ID_GLOBAL_POSITION_INT and MAVLINK_MSG_ID_SCALED_PRESSUREs
@@ -472,9 +469,8 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
                     }
                 }
             }
-        }
-        // Else its from the GCS, and it might be for the remote and.or it might be for the tracker
-        // So we fall through to the below
+        // no further processing of messages from vehicle
+        return;
     }