Remove unnecessary def_map copying.

Introduced in 5b080bacc4
Made obsolete by 27b9ddda5b
[Context](https://github.com/google-deepmind/mujoco/commit/27b9ddda5bdd12345abdfbc0a6efce121d40cd6c#r144157607)

PiperOrigin-RevId: 653254345
Change-Id: Ie944d2c3924d3eb91f7743c0d0c8413cd2cd79cc

src/user/user_model.cc

@@ -136,12 +136,6 @@ mjCModel& mjCModel::operator=(const mjCModel& other) {
     // create new default tree
     mjCDef* subtree = new mjCDef(*other.defaults_[0]);
     *this += *subtree;
-    for (const auto& [name, def] : other.def_map) {
-      std::size_t index =
-          std::find(other.defaults_.begin(), other.defaults_.end(), def) -
-          other.defaults_.begin();
-      def_map[name] = defaults_[index];
-    }
 
     // copy name maps
     for (int i=0; i<mjNOBJECT; i++) {