AP_Mount: correct compiler warnings

In file included from ../../libraries/AP_Mount/SoloGimbal.cpp:5:
../../libraries/AP_Mount/SoloGimbal.h:122:11: warning: private field
'_yaw_rate_ff_ef_filt' is not used [-Wunused-private-field]
    float _yaw_rate_ff_ef_filt;
          ^
../../libraries/AP_Mount/SoloGimbal.h:140:23: warning: private field
'_chan' is not used [-Wunused-private-field]
    mavlink_channel_t _chan;
                      ^
2 warnings generated.

libraries/AP_Mount/SoloGimbal.h

@@ -42,7 +42,6 @@ public:
         _ekf(ahrs),
         _ahrs(ahrs),
         _state(GIMBAL_STATE_NOT_PRESENT),
-        _yaw_rate_ff_ef_filt(0.0f),
         _vehicle_yaw_rate_ef_filt(0.0f),
         _vehicle_to_gimbal_quat(),
         _vehicle_to_gimbal_quat_filt(),
@@ -119,7 +118,6 @@ private:
         Vector3f joint_angles;
     } _measurement;
 
-    float _yaw_rate_ff_ef_filt;
     float _vehicle_yaw_rate_ef_filt;
 
     static const uint8_t _compid = MAV_COMP_ID_GIMBAL;
@@ -137,8 +135,6 @@ private:
 
     float _ang_vel_mag_filt;
 
-    mavlink_channel_t _chan;
-
     Vector3f    _ang_vel_dem_rads;       // rad/s
     Vector3f    _att_target_euler_rad;   // desired earth-frame roll, tilt and pan angles in radians