Remove obsolete expire_auth_token_on_timeout

This should have been removed in dff7891b97
when token authentication (and the reset_authentication_token! method)
were removed.

test/rails_app/app/controllers/admins_controller.rb

@@ -3,9 +3,4 @@ class AdminsController < ApplicationController
 
   def index
   end
-
-  def expire
-    admin_session['last_request_at'] = 31.minutes.ago.utc
-    render text: 'Admin will be expired on next request'
-  end
 end