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 773ee47.

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