Include obj and stl decoder plugins as sources in CMake builds.

This removes the need to load these decoders via mj_loadAllPluginLibraries when using MuJoCo built with CMake. Other plugins are unchanged.

PiperOrigin-RevId: 897114719
Change-Id: Iee914cc5e05798186f384a67901f1cf86bc2f887

sample/testspeed.cc

@@ -187,12 +187,6 @@ int main(int argc, char** argv) {
   nthread = mjMAX(1, mjMIN(maxthread, nthread));
   npoolthread = mjMAX(1, mjMIN(maxthread, npoolthread));
 
-  // load plugins from MUJOCO_PLUGIN_DIR if set
-  const char* plugin_dir = std::getenv("MUJOCO_PLUGIN_DIR");
-  if (plugin_dir) {
-    mj_loadAllPluginLibraries(plugin_dir, nullptr);
-  }
-
   // get filename, determine file type
   std::string filename(argv[1]);
   bool binary = (filename.find(".mjb") != std::string::npos);  // NOLINT