DataFlash: only log two rangefinders

this prevents a array index build error on PX4

libraries/DataFlash/DataFlash.h

@@ -533,8 +533,6 @@ struct PACKED log_RFND {
     uint64_t time_us;
     uint16_t dist1;
     uint16_t dist2;
-    uint16_t dist3;
-    uint16_t dist4;
 };
 
 /*
@@ -761,7 +759,7 @@ Format characters in the format string for binary log messages
     { LOG_MODE_MSG, sizeof(log_Mode), \
       "MODE", "QMB",         "TimeUS,Mode,ModeNum" }, \
     { LOG_RFND_MSG, sizeof(log_RFND), \
-      "RFND", "QCCCC",         "TimeUS,Dist1,Dist2,Dist3,Dist4" }
+      "RFND", "QCC",         "TimeUS,Dist1,Dist2" }
 
 // messages for more advanced boards
 #define LOG_EXTRA_STRUCTURES \

libraries/DataFlash/LogFile.cpp

@@ -755,9 +755,7 @@ void DataFlash_Class::Log_Write_RFND(const RangeFinder &rangefinder)
         LOG_PACKET_HEADER_INIT((uint8_t)(LOG_RFND_MSG)),
         time_us       : hal.scheduler->micros64(),
         dist1         : rangefinder.distance_cm(0),
-        dist2         : rangefinder.distance_cm(1),
-        dist3         : rangefinder.distance_cm(2),
-        dist4         : rangefinder.distance_cm(3)
+        dist2         : rangefinder.distance_cm(1)
     };
     WriteBlock(&pkt, sizeof(pkt));
 }