Removed the unused require of proxy_object and test related to it

- Reference : https://github.com/rails/rails/pull/17493#issuecomment-61739359
- Duration stopped inheriting from ProxyObject in https://github.com/rails/rails/pull/16574

activesupport/lib/active_support/duration.rb

@@ -1,4 +1,3 @@
-require 'active_support/proxy_object'
 require 'active_support/core_ext/array/conversions'
 require 'active_support/core_ext/object/acts_like'
 

activesupport/test/core_ext/duration_test.rb

@@ -30,7 +30,6 @@ class DurationTest < ActiveSupport::TestCase
     assert ActiveSupport::Duration === 1.day
     assert !(ActiveSupport::Duration === 1.day.to_i)
     assert !(ActiveSupport::Duration === 'foo')
-    assert !(ActiveSupport::Duration === ActiveSupport::ProxyObject.new)
   end
 
   def test_equals