AP_Proximity: compiler warning fixes

In file included from
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.cpp:30:
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.h:88:10: warning:
private field '_payload_data' is not used [-Wunused-private-field]
    bool _payload_data;
         ^
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.h:91:10: warning:
private field '_skip' is not used [-Wunused-private-field]
    bool _skip;
         ^
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.h:92:10: warning:
private field '_rp_reset' is not used [-Wunused-private-field]
    bool _rp_reset;
         ^
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.h:95:13: warning:
private field '_element_len' is not used [-Wunused-private-field]
    uint8_t _element_len[2];
            ^
../../libraries/AP_Proximity/AP_Proximity_RPLidarA2.h:96:13: warning:
private field '_element_num' is not used [-Wunused-private-field]
    uint8_t _element_num;
            ^
5 warnings generated.

libraries/AP_Proximity/AP_Proximity_RPLidarA2.h

@@ -85,15 +85,10 @@ private:
     char _rp_systeminfo[63];
     bool _descriptor_data;
     bool _information_data;
-    bool _payload_data;
     bool _resetted;
     bool _initialised;
-    bool _skip;
-    bool _rp_reset;
     bool _sector_initialised;
 
-    uint8_t _element_len[2];
-    uint8_t _element_num;
     uint8_t _payload_length;
     uint8_t _cnt;
     uint8_t _sync_error ;