Testrunner has ancient support for JUnit compatible XML output.

This CL removes this old feature.

R=mstarzinger@chromium.org,jgruber@chromium.org,jkummerow@chromium.org
CC=​machenbach@chromium.org

Bug: v8:8728
Change-Id: I7e1beb011dbaec3aa1a27398a5c52abdd778eaf0
Reviewed-on: https://chromium-review.googlesource.com/c/1430065Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59045}