Remove `frozen_string_literal` magic comment from template file

The other template files do not add `frozen_string_literal`, so should
behave the same.

Ref: #30342, #30348.

actionmailbox/lib/rails/generators/test_unit/templates/mailbox_test.rb.tt

-# frozen_string_literal: true
-
 require "test_helper"
 
 class <%= class_name %>MailboxTest < ActionMailbox::TestCase