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

Pacemaker requires libxml2, so we can reasonably count on xmllint being
available. Other RelaxNG validators may have their own advantages and
disadvantages (for example, jing has much better error messages).
However, Pacemaker uses libxml2 internally, so that's all we need to
support in regression.sh. Support for other validators is just a
maintenance burden.

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