Remove config.gem deprecation stub since its not "deprecated" but

completely removed.

This makes feature detection easier with config.respond_to?(:gem)

railties/lib/rails/deprecation.rb

@@ -15,11 +15,3 @@ RAILS_ROOT = (Class.new(ActiveSupport::Deprecation::DeprecationProxy) do
     ActiveSupport::Deprecation.warn(msg, callstack)
   end
 end).new
-
-module Rails
-  class Configuration
-    def gem(*args)
-      ActiveSupport::Deprecation.warn("config.gem has been deprecated in favor of the Gemfile.")
-    end
-  end
-end