Documentation: remove Asciidoctor linkgit macro

Asciidoctor provides an extension implementing a backend-independent
macro for dealing with manpage links just like the linkgit macro.  As
this is more likely to be up-to-date with future changes in Asciidoctor,
prefer using it over reimplementing in Git.

This reverts commit 773ee47c2b.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/extensions.rb

@@ -1,39 +0,0 @@
-require 'asciidoctor'
-require 'asciidoctor/extensions'
-
-module Git
-  module Documentation
-    class LinkGitProcessor < Asciidoctor::Extensions::InlineMacroProcessor
-      use_dsl
-
-      named :chrome
-
-      def process(parent, target, attrs)
-        if parent.document.basebackend? 'html'
-          generate_html(parent, target, attrs)
-        elsif parent.document.basebackend? 'docbook'
-          generate_docbook(parent, target, attrs)
-        end
-      end
-
-      private
-
-      def generate_html(parent, target, attrs)
-        section = attrs.has_key?(1) ? "(#{attrs[1]})" : ''
-        prefix = parent.document.attr('git-relative-html-prefix') || ''
-        %(<a href="#{prefix}#{target}.html">#{target}#{section}</a>\n)
-      end
-
-      def generate_docbook(parent, target, attrs)
-        %(<citerefentry>
-<refentrytitle>#{target}</refentrytitle><manvolnum>#{attrs[1]}</manvolnum>
-</citerefentry>
-)
-      end
-    end
-  end
-end
-
-Asciidoctor::Extensions.register do
-  inline_macro Git::Documentation::LinkGitProcessor, :linkgit
-end