Remove requires from email handlers

ee/lib/ee/gitlab/email/handler.rb

 # frozen_string_literal: true
 
-require 'gitlab/email/handler/ee/service_desk_handler'
-
 module EE
   module Gitlab
     module Email

lib/gitlab/email/handler.rb

 # frozen_string_literal: true
 
-require 'gitlab/email/handler/create_merge_request_handler'
-require 'gitlab/email/handler/create_note_handler'
-require 'gitlab/email/handler/create_issue_handler'
-require 'gitlab/email/handler/unsubscribe_handler'
-
-require 'ee/gitlab/email/handler'
-
 module Gitlab
   module Email
     module Handler