Refactor: xml: Drop -S (self-check) option from xml/regression.sh

All it does is validate the transforms themselves against an XSLT
RelaxNG schema. If they're invalid, the transforms should simply fail.
The self-check test may be useful for debugging in limited
circumstances, but it's not worth the maintenance headache, especially
as we reimplement regression testing in Python.

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