Remove the GFM parser; it makes no sense

The new versions of GitHub::Markup will automatically render GFM for
Markdown documents if the necessary gems are installed.

lib/gollum/markup.rb

@@ -484,64 +484,4 @@ module Gollum
     def update_cache(type, id, data)
     end
   end
-
-  begin
-    require 'redcarpet'
-
-    class MarkupGFM < Markup
-      def render(no_follow = false, encoding = nil)
-        sanitize = no_follow ?
-          @wiki.history_sanitizer :
-          @wiki.sanitizer
-
-        data = extract_tex(@data.dup)
-        data = extract_code(data)
-        data = extract_tags(data)
-
-        if Gem::Version.new(Redcarpet::VERSION) > Gem::Version.new("1.17.2")
-          html_renderer = Redcarpet::Render::HTML.new({
-            :autolink => true,
-            :fenced_code_blocks => true,
-            :tables => true,
-            :strikethrough => true,
-            :lax_htmlblock => true,
-            :no_intraemphasis => true
-          })
-          markdown = Redcarpet::Markdown.new(html_renderer)
-          data = markdown.render(data)
-        else
-          flags = [
-            :autolink,
-            :fenced_code,
-            :tables,
-            :strikethrough,
-            :lax_htmlblock,
-            :no_intraemphasis
-          ]
-          data = Redcarpet.new(data, *flags).to_html
-        end
-        data = process_tags(data)
-        data = process_code(data, encoding)
-
-        doc  = Nokogiri::HTML::DocumentFragment.parse(data)
-
-        doc.search('pre').each do |node|
-          next unless lang = node['lang']
-          next unless lexer = Pygments::Lexer[lang]
-          text = node.inner_text
-          html = lexer.highlight(text)
-          node.replace(html)
-        end
-
-        doc  = sanitize.clean_node!(doc) if sanitize
-        yield doc if block_given?
-
-        data = doc.to_html
-        data = process_tex(data)
-        data
-      end
-    end
-  rescue LoadError
-    MarkupGFM = Markup
-  end
 end

test/test_markup.rb

@@ -456,11 +456,6 @@ np.array([[2,2],[1,3]],np.float)
     # rendered with Gollum::Markup
     page, rendered = render_page(content)
     assert_markup_highlights_code Gollum::Markup, rendered
-
-    if Gollum.const_defined?(:MarkupGFM)
-      rendered_gfm = Gollum::MarkupGFM.new(page).render
-      assert_markup_highlights_code Gollum::MarkupGFM, rendered_gfm
-    end
   end
 
   def assert_markup_highlights_code(markup_class, rendered)