docs: Remove Markdown from configure.ac and the build instructions

Now that all use of markdown as been replaced with pandoc, drop it from
configure.ac and from the docs/ Makefile.

Instead of just deleting the markdown reference from README, adjust the line
to mention the other optional packages required for documentation.

Drop markdown from travis, but leave it in the docker containers as they get
used for stable branches as well.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

config/Docs.mk.in

@@ -5,6 +5,5 @@ FIG2DEV             := @FIG2DEV@
 POD2MAN             := @POD2MAN@
 POD2HTML            := @POD2HTML@
 POD2TEXT            := @POD2TEXT@
-MARKDOWN            := @MARKDOWN@
 PANDOC              := @PANDOC@
 PERL                := @PERL@