tools/patchwork_gitlab_igt: Remove undocumented test handling

Minimal changes needed to strip them out. Tracking the last postmerge
is not needed anymore and can be taken out later.

Arek's changes with
drm/igt-gpu-tools#99

 removes
the need to handle the comparison so this script can just report the
pipeline status to patchwork.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>