Remove dead `Gitlab::Markdown::Pipeline` class

lib/gitlab/markdown/pipeline.rb

-module Gitlab
-  module Markdown
-    class Pipeline
-      def self.[](name)
-        name ||= :full
-        const_get("#{name.to_s.camelize}Pipeline")
-      end
-
-      def self.filters
-        []
-      end
-
-      def self.transform_context(context)
-        context
-      end
-
-      def self.html_pipeline
-        @html_pipeline ||= HTML::Pipeline.new(filters)
-      end
-
-      class << self
-        %i(call to_document to_html).each do |meth|
-          define_method(meth) do |text, context|
-            context = transform_context(context)
-
-            html_pipeline.__send__(meth, text, context) # rubocop:disable GitlabSecurity/PublicSend
-          end
-        end
-      end
-    end
-  end
-end