Raise error when binding to a removed element.

PiperOrigin-RevId: 774462737
Change-Id: I820c0e1c010ae9fbf6d9667c3364d4b65bf21002

src/user/user_model.h

@@ -229,9 +229,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   // delete object from the corresponding list
   void operator-=(mjsElement* el);
 
-  // delete object
-  void DeleteElement(mjsElement* el);
-
   // delete default and all descendants
   void RemoveDefault(mjCDef* def);