JSHint vertical-tabs.js

Follow-up of #1664940: [Policy, patch] Decide on JSHint configuration and part of #1415788: Javascript winter clean-up

Run jshint on the files with the configuration from the parent issue or use jshint.com with the following options:

/*jshint forin:true, noarg:true, eqeqeq:true, undef:true, curly:true, browser:true, expr:true, latedef:true, newcap:true, trailing:true */
/*global Drupal, jQuery */

Fix any warnings or errors the tool finds.
Check manually that the fixes did not break any functionalities
Create patch and upload for the testbot.

Files: vertical-tabs.js