Revert "Add missing require "active_support in XmlMini"

This reverts commit 0181f0e.

Users of Active Support should always require the top level file of
the framework before requiring anything else inside it, so we don't
need to require that top level file here.

Author: rafaelfranca

activesupport/lib/active_support/xml_mini.rb

@@ -4,7 +4,6 @@
 require "base64"
 require "bigdecimal"
 require "bigdecimal/util"
-require "active_support"
 require "active_support/core_ext/module/delegation"
 require "active_support/core_ext/string/inflections"
 require "active_support/core_ext/date_time/calculations"