Remove jquery_ujs from main.js manifest

Since jquery and jquery_ujs are now included by the jquery_include_tag
helper (see commit 9e752585), they shouldn't be included by the asset
pipeline in our javascript manifests. :)

app/assets/javascripts/main.js

@@ -2,7 +2,6 @@
  *   licensed under the Affero General Public License version 3 or later.  See
  *   the COPYRIGHT file.
  */
-//= require jquery_ujs
 //= require underscore
 //= require backbone
 //= require rails.validations