Avoid Mocha deprecation warning in test helper

This avoids the following deprecation warning:

    Mocha deprecation warning at test/test_helper.rb:34:in `block in
    <main>': Configuration#reinstate_undocumented_behaviour_from_v1_9=
    is unnecessarily being set to false, because this is now the default
    value. Configuration#reinstate_undocumented_behaviour_from_v1_9=
    will be removed in the future, so you should avoid calling it.

test/test_helper.rb

@@ -31,7 +31,6 @@
 Whitehall::Application.load_tasks if Rake::Task.tasks.empty?
 
 Mocha.configure do |c|
-  c.reinstate_undocumented_behaviour_from_v1_9 = false
   c.stubbing_non_existent_method = :prevent
 end