This removes some obsolete HAML:

* In app/views/ci/variables/_variable_row.html.haml, some code was
  guarded by a condition that always evaluates to false and has thus
  been removed.
* The app/views/shared/buttons/_project_feature_toggle.html.haml view
  has been deleted as it's not used anywhere anymore. From now on, we
  should use the Pajamas-complient _gl_toggle.html.haml view.