Do not expose plugin_slot.

PiperOrigin-RevId: 683215401
Change-Id: Ic64f7221ed83ca3b0c8cca16eb680ea67c67b564

include/mujoco/mjspec.h

@@ -182,9 +182,8 @@ typedef struct mjsOrientation_ {   // alternative orientation specifiers
 
 typedef struct mjsPlugin_ {        // plugin specification
   mjsElement* element;             // element type
-  mjString* name;                  // name
+  mjString* name;                  // instance name
   mjString* plugin_name;           // plugin name
-  int plugin_slot;                 // global registered slot number of the plugin
   mjtByte active;                  // is the plugin active
   mjString* info;                  // message appended to compiler errors
 } mjsPlugin;

include/mujoco/mujoco.h

@@ -241,8 +241,8 @@ MJAPI mjSpec* mj_copySpec(const mjSpec* s);
 // Free memory allocation in mjSpec.
 MJAPI void mj_deleteSpec(mjSpec* s);
 
-// Activate plugin, return slot number.
-MJAPI int mjs_activatePlugin(mjSpec* s, const char* name);
+// Activate plugin.
+MJAPI void mjs_activatePlugin(mjSpec* s, const char* name);
 
 
 //---------------------------------- Printing ------------------------------------------------------