unregister(callback) {
    if (this.pluginCallbacks.indexOf(callback) !== -1) {
      this.pluginCallbacks.splice(this.pluginCallbacks.indexOf(callback), 1);
    }
    return this;
  }
Unregisters a plugin callback.

@param {Function} callback - The callback function to unregister.
@return {GLTFLoader} A reference to this loader.