remove deprication warning for ruby 1.9.3-head for unused variables

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -577,7 +577,7 @@ def limited_update_conditions(where_sql, quoted_table_name, quoted_primary_key)
         def quoted_columns_for_index(column_names, options = {})
           length = options[:length] if options.is_a?(Hash)
 
-          quoted_column_names = case length
+          case length
           when Hash
             column_names.map {|name| length[name] ? "#{quote_column_name(name)}(#{length[name]})" : quote_column_name(name) }
           when Fixnum