Revert "Set x-frame-option to sameorigin to allow the Sidekiq iframe to display."

This reverts commit 754b0838.

Sidekiq rendered via mounted sinatra app. We don't need to change
controller headers. It won't affect sidekiq at all. Please modify nginx
config instead for all gitlab app.

CHANGELOG

@@ -13,7 +13,6 @@ v 7.0.0
   - Group masters can create projects in group
   - Deprecate ruby 1.9.3 support
   - Only masters can rewrite/remove git tags
-  - Header X-Frame-Options allows SAMEORIGIN to display the Sidekiq interface
 
 v 6.9.2
   - Revert the commit that broke the LDAP user filter

app/controllers/application_controller.rb

@@ -164,7 +164,7 @@ class ApplicationController < ActionController::Base
   end
 
   def default_headers
-    headers['X-Frame-Options'] = 'SAMEORIGIN' # Allow for the Sidekiq iframe in /admin/background_jobs
+    headers['X-Frame-Options'] = 'DENY'
     headers['X-XSS-Protection'] = '1; mode=block'
     headers['X-UA-Compatible'] = 'IE=edge'
     headers['X-Content-Type-Options'] = 'nosniff'