Remove promise jsdoc (#641)

This patch removes unnecessary "promise" return types in jsdoc for async functions.

lib/helper.js

@@ -90,7 +90,6 @@ class Helper {
   /**
    * @param {!Session} client
    * @param {!Object} remoteObject
-   * @return {!Promise}
    */
   static async releaseObject(client, remoteObject) {
     if (!remoteObject.objectId)