Remove frozen_string_literal pragma from Action Mailbox templates

actionmailbox/lib/rails/generators/mailbox/templates/application_mailbox.rb.tt

-# frozen_string_literal: true
-
 class ApplicationMailbox < ActionMailbox::Base
   # routing /something/i => :somewhere
 end

actionmailbox/lib/rails/generators/mailbox/templates/mailbox.rb.tt

-# frozen_string_literal: true
-
 class <%= class_name %>Mailbox < ApplicationMailbox
   def process
   end