Plane: Remove ALT_MIX

Closes #4998, and fix a tab/spaces error on the previous parameter

ArduPlane/Parameters.cpp

@@ -242,15 +242,6 @@ const AP_Param::Info Plane::var_info[] = {
     // @User: Standard
     GSCALAR(nav_controller,          "NAV_CONTROLLER",   AP_Navigation::CONTROLLER_L1),
 
-    // @Param: ALT_MIX
-    // @DisplayName: GPS to Baro Mix
-    // @Description: The percent of mixing between GPS altitude and baro altitude. 0 = 100% gps, 1 = 100% baro. It is highly recommend that you not change this from the default of 1, as GPS altitude is notoriously unreliable. The only time I would recommend changing this is if you have a high altitude enabled GPS, and you are dropping a plane from a high altitude balloon many kilometers off the ground.
-    // @Units: Percent
-    // @Range: 0 1
-    // @Increment: 0.1
-    // @User: Advanced
-    GSCALAR(altitude_mix,           "ALT_MIX",        ALTITUDE_MIX),
-
     // @Param: ALT_CTRL_ALG
     // @DisplayName: Altitude control algorithm
     // @Description: This sets what algorithm will be used for altitude control. The default is zero, which selects the most appropriate algorithm for your airframe. Currently the default is to use TECS (total energy control system). From time to time we will add other experimental altitude control algorithms which will be selected using this parameter.

ArduPlane/Parameters.h

@@ -189,7 +189,7 @@ public:
         // 130: Sensor parameters
         //
         k_param_imu = 130,  // unused
-        k_param_altitude_mix,
+        k_param_altitude_mix, // deprecated
 
         k_param_compass_enabled,
         k_param_compass,
@@ -393,7 +393,6 @@ public:
 
     // Estimation
     //
-    AP_Float altitude_mix;
     AP_Int8  alt_control_algorithm;
 
     // Waypoints