AP_GPS_GSOF: remove unused last_hdop

resolves a compiler warning

libraries/AP_GPS/AP_GPS_GSOF.h

@@ -83,7 +83,6 @@ private:
     uint8_t gsofmsgreq_index = 0;
     uint8_t gsofmsgreq[5] = {1,2,8,9,12};
 
-    uint32_t last_hdop = 9999;
     uint32_t crc_error_counter = 0;
     uint32_t last_injected_data_ms = 0;
 };