Remove support for old categories.json file syntax.

      def generate

        @categories = Util::Json.read(@filename)

        # Don't crash if old syntax is used.

        if @categories.is_a?(Hash) && @categories["categories"]

          Logger.warn(nil, 'Update categories file to contain just the array inside {"categories": [...]}', @filename)

          @categories = @categories["categories"]

        end

        # Perform expansion on all class names containing * wildcard

        @categories.each do |cat|

          cat["groups"].each do |group|