Move kinematic functions to "private" scope

Marlin/Marlin.h

   extern float delta_diagonal_rod_trim_tower_3;
   void calculate_delta(float cartesian[3]);
   void recalc_delta_settings(float radius, float diagonal_rod);
-  float delta_safe_distance_from_top();
-  void set_cartesian_from_steppers();
-  void forwardKinematics(float point[3]);
-  void forwardKinematics(float z1, float z2, float z3);
   #if ENABLED(AUTO_BED_LEVELING_FEATURE)
     extern int delta_grid_spacing[2];
     void adjust_delta(float cartesian[3]);

Marlin/Marlin_main.cpp

     int delta_grid_spacing[2] = { 0, 0 };
     float bed_level[AUTO_BED_LEVELING_GRID_POINTS][AUTO_BED_LEVELING_GRID_POINTS];
   #endif
+  float delta_safe_distance_from_top();
 #else
   static bool home_all_axis = true;
 #endif