Remove Docbook conversion in tox for HA Guide

The High Availability Guide is currently written in ASCIIdoc, so
the tox scripts have to convert it to Docbook before continuing to
test. Remove all special casing needed for this.

Co-Authored-By: Andreas Jaeger <aj@suse.com>
Change-Id: I5a6c13e502d003237eb93d27028de2cb9db02e04
Implements: blueprint convert-ha-guide-to-docbook