Build: xml: remove all generated pacemaker-*.rng schema files for clean

Previously, "make clean" in the xml directory would remove the pacemaker-*.rng
files that were generated by the current source version. That works fine as
long as you only move forward in a checkout. If you checked out an earlier
revision after a newer one had already been built, "make clean" would not
remove newer schema files than the checked-out version created, which would
cause processing errors when binaries searched the schema directory and tried
to upgrade to the newest version.