drm/xe/kunit: Drop xe_wa tests for pre-production DG2

As workarounds for pre-production DG2 were dropped in
commit 707d1b992cfe ("drm/xe/dg2: Drop pre-production workarounds"),
there's no point running the kunit tests for them. Drop
those steppings from kunit.

Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20231221163213.3849523-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>