Copter: remove climb_rate cache of inertial_nav.get_velocity_z

There were only two users of it and dozens of places using
get_velocity_z

ArduCopter/Attitude.cpp

@@ -56,7 +56,7 @@ void Copter::update_throttle_hover()
     float throttle = motors->get_throttle();
 
     // calc average throttle if we are in a level hover
-    if (throttle > 0.0f && abs(climb_rate) < 60 && labs(ahrs.roll_sensor) < 500 && labs(ahrs.pitch_sensor) < 500) {
+    if (throttle > 0.0f && abs(inertial_nav.get_velocity_z()) < 60 && labs(ahrs.roll_sensor) < 500 && labs(ahrs.pitch_sensor) < 500) {
         // Can we set the time constant automatically
         motors->update_throttle_hover(0.01f);
     }

ArduCopter/Copter.h

@@ -402,8 +402,6 @@ private:
 #endif
 
     // Altitude
-    // The cm/s we are moving up or down based on filtered data - Positive = UP
-    int16_t climb_rate;
     float target_rangefinder_alt;   // desired altitude in cm above the ground
     bool target_rangefinder_alt_used; // true if mode is using target_rangefinder_alt
     int32_t baro_alt;            // barometer altitude in cm above home

ArduCopter/Log.cpp

@@ -59,7 +59,7 @@ void Copter::Log_Write_Control_Tuning()
         rangefinder_alt     : rangefinder_state.alt_cm,
         terr_alt            : terr_alt,
         target_climb_rate   : target_climb_rate_cms,
-        climb_rate          : climb_rate
+        climb_rate          : int16_t(inertial_nav.get_velocity_z()) // float -> int16_t
     };
     logger.WriteBlock(&pkt, sizeof(pkt));
 }

ArduCopter/inertia.cpp

@@ -24,7 +24,4 @@ void Copter::read_inertia()
     }
     current_loc.set_alt_cm(inertial_nav.get_altitude(), frame);
     current_loc.change_alt_frame(Location::AltFrame::ABOVE_HOME);
-
-    // set flags and get velocity
-    climb_rate = inertial_nav.get_velocity_z();
 }