Remove a useless constant

It seems that unnecessary constant was left behind when introducing the Inject mechanism,
inspired by RuboCop RSpec, during the initial implementation of this gem.

Author: koic

lib/rubocop/rails.rb

@@ -5,7 +5,6 @@ module RuboCop
   module Rails
     PROJECT_ROOT   = Pathname.new(__dir__).parent.parent.expand_path.freeze
     CONFIG_DEFAULT = PROJECT_ROOT.join('config', 'default.yml').freeze
-    CONFIG         = YAML.safe_load(CONFIG_DEFAULT.read, permitted_classes: [Regexp, Symbol]).freeze
 
     private_constant(:CONFIG_DEFAULT, :PROJECT_ROOT)