doc: remove custom callouts format

doc: remove custom callouts format

The code to render callouts for manpages comes from 17 years ago:
776e994af5 (Properly render asciidoc "callouts" in git man pages.,
2006-04-28), and it was needed back then, but DocBook Stylesheets added
support for that in 2008 [1], since 1.74.0 it hasn't been necessary.

What's worse: the format of the upstream callouts is much nicer than our
hacked version.

Compare this:

     $ git diff            (1)
     $ git diff --cached   (2)
     $ git diff HEAD       (3)

  1. Changes in the working tree not yet staged for the next
     commit.
  2. Changes between the index and your last commit; what you
     would be committing if you run git commit without -a
     option.
  3. Changes in the working tree since your last commit; what
     you would be committing if you run git commit -a

To this:

     $ git diff            (1)
     $ git diff --cached   (2)
     $ git diff HEAD       (3)

 1. Changes in the working tree not yet staged for the next commit.
 2. Changes between the index and your last commit; what you would
 be committing if you run git commit without -a option.
 3. Changes in the working tree since your last commit; what you
 would be committing if you run git commit -a

Let's drop our unnecessary inferior custom format and use the official
one.

[1] https://sourceforge.net/p/docbook/code/7842/

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>