ci/trace: Move trace cleanup to Piglit runner

No sense in polluting our common init code with this.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31110>

.gitlab-ci/piglit/piglit-traces.sh

@@ -219,6 +219,7 @@ fi
 
 
 if [ ! -s $RESULTSFILE ]; then
+    rm -rf "${RESULTS_DIR:?}/${__PREFIX}"
     exit 0
 fi