Remove copy_back from Python bindings and mj_copyBack from the C API.

PiperOrigin-RevId: 687377331
Change-Id: I355493d92857418b08b5aa90ea38cebe5c475314

src/xml/xml_base.h

@@ -88,7 +88,7 @@ class mjXBase : public mjXUtil {
   };
 
   // set the model allocated externally
-  virtual void SetModel(const mjSpec*);
+  virtual void SetModel(const mjSpec*, const mjModel* = nullptr);
 
   // read alternative orientation specification
   static int ReadAlternative(tinyxml2::XMLElement* elem, mjsOrientation& alt);