Add unregister_template_handler to prevent leaks.

A few places in the test codebase had `ActionView::Template.register_template_handler`, but the handlers were not removed after the tests.

This patch adds a new method `unregister_template_handler` to `ActionView::Template::Handlers`, and makes sure related tests are cleaned up properly.

cc @senny
This code was first added in 7c0dfa35, together with `actionmailer/test/mail_service_test.rb`. The file was then moved to `actionmailer/test/old_base/mail_service_test.rb` in bd966141, and then deleted in d500ad3f. However, these three lines of code are left behind and in fact not used anywhere.