Cleanup constexpr in planner.cpp

Marlin/planner.cpp

     float Planner::xy_skew_factor;
     #if ENABLED(SKEW_CORRECTION_FOR_Z)
       float Planner::xz_skew_factor, Planner::yz_skew_factor;
-    #else
-      constexpr float Planner::xz_skew_factor, Planner::yz_skew_factor;
     #endif
-  #else
-    constexpr float Planner::xy_skew_factor, Planner::xz_skew_factor, Planner::yz_skew_factor;
   #endif
 #endif