Removed unused commented code [ci skip]

As discussed in #13940 this can be removed
We can document this somewhere else in testing guides.

guides/source/testing.md

@@ -937,7 +937,6 @@ Here's a unit test to test a mailer named `UserMailer` whose action `invite` is
 require 'test_helper'
 
 class UserMailerTest < ActionMailer::TestCase
-  # tests UserMailer
   test "invite" do
     # Send the email, then test that it got queued
     email = UserMailer.create_invite('me@example.com',