Test: cts-cli: Drop Relax-NG validity error prefixes

Tests are currently breaking on debian-experimental (which uses a newer
version of libxml2) because this prefix does not appear in the output.

This change should ensure the validity errors appear the same on all
platforms (as long as the message itself doesn't change).

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