not needed require's

- as core_ext is not used and test pass locally
- mail is already required in abstract_unit

actionmailer/test/delivery_methods_test.rb

 require 'abstract_unit'
-require 'mail'
 
 class MyCustomDelivery
 end

actionmailer/test/message_delivery_test.rb

@@ -2,7 +2,6 @@
 require 'active_job'
 require 'minitest/mock'
 require 'mailers/delayed_mailer'
-require 'active_support/core_ext/numeric/time'
 
 class MessageDeliveryTest < ActiveSupport::TestCase
   include ActiveJob::TestHelper