Feature: xml: Drop XSLTPROCESSOR environment variable from regression.sh

Pacemaker requires libxslt, so we can reasonably count on xsltproc being
available. Other XSLT processors may have their own advantages and
disadvantages. However, Pacemaker uses libxslt internally, so that's all
we need to support in regression.sh. Support for the other processors is
just a maintenance burden.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>