Revert "Missing requires"

These rely on constant autoloads.

This reverts commit d39f397dc6.

activesupport/lib/active_support/callbacks.rb

@@ -1,7 +1,6 @@
 require 'active_support/core_ext/array/wrap'
 require 'active_support/core_ext/class/inheritable_attributes'
 require 'active_support/core_ext/kernel/reporting'
-require 'active_support/concern'
 
 module ActiveSupport
   # Callbacks are hooks into the lifecycle of an object that allow you to trigger logic

activesupport/lib/active_support/testing/setup_and_teardown.rb

@@ -1,4 +1,3 @@
-require 'active_support/callbacks'
 module ActiveSupport
   module Testing
     module SetupAndTeardown