Add OnePlugin read/write functions and remove redundant plugin attributes from mjCActuator.

PiperOrigin-RevId: 525708657
Change-Id: I762de238f989b13877d5f9198c83c16ea9dc1d8b

src/user/user_objects.h

@@ -929,12 +929,6 @@ class mjCActuator : public mjCBase {
   std::string slidersite;         // site defining cylinder, for slider-crank only
   std::string refsite;            // reference site, for site transmission only
 
-  // plugin support
-  bool is_plugin;
-  std::string plugin_name;
-  std::string plugin_instance_name;
-  mjCPlugin* plugin_instance;
-
  private:
   mjCActuator(mjCModel* = 0, mjCDef* = 0);// constructor
   void Compile(void);                     // compiler