AP_Winch: remove redundant member

Also use get_rate_max method instead of config.rate_max member

libraries/AP_Winch/AP_Winch.cpp

@@ -113,7 +113,7 @@ void AP_Winch::release_length(float length, float rate)
     if (is_zero(rate)) {
         config.rate_desired = config.rate_max;
     } else {
-        config.rate_desired = constrain_float(fabsf(rate), -config.rate_max, config.rate_max);
+        config.rate_desired = constrain_float(fabsf(rate), -get_rate_max(), get_rate_max());
     }
 }
 

libraries/AP_Winch/AP_Winch_Servo.cpp

@@ -40,9 +40,8 @@ void AP_Winch_Servo::update()
     float distance = _wheel_encoder->get_distance(0);
     float rate = 0.0f;
     if (is_positive(dt)) {
-        rate = (distance - _last_distance) / dt;
+        rate = (distance - config.length_curr) / dt;
     }
-    _last_distance = distance;
 
     // update distance from wheel encoder
     config.length_curr = distance;

libraries/AP_Winch/AP_Winch_Servo.h

@@ -34,7 +34,6 @@ private:
     // external reference
     const AP_WheelEncoder* _wheel_encoder;
 
-    float _last_distance;       // wheel encoder total distance from previous iteration (used to calculate rate)
     uint32_t last_update_ms;    // last time update was called
     bool limit_high;            // output hit limit on last iteration
     bool limit_low;             // output hit lower limit on last iteration