Remove the _cmp metamethod from the entity types because it's useless for registered types.

source/Entity/Vehicle.hpp

@@ -105,11 +105,6 @@ public:
         }
     }
 
-    /* --------------------------------------------------------------------------------------------
-     * Used by the script engine to compare two instances of this type.
-    */
-    Int32 Cmp(const CVehicle & o) const;
-
     /* --------------------------------------------------------------------------------------------
      * Used by the script engine to convert an instance of this type to a string.
     */