Fix deletion of stale patch sets in Gerrit-verifier pipeline

The configuration for removing old patch sets in the Gerrit-verifier
pipeline was erroneously set to disabled. Thus, a huge number of old
job data was kept for a long time.

Change-Id: I6aec3e16c40210ff7befba585c89493f3a8936da
diff --git a/jenkins/gerrit-verifier.yaml b/jenkins/gerrit-verifier.yaml
index eefcdc8..699e5f4 100644
--- a/jenkins/gerrit-verifier.yaml
+++ b/jenkins/gerrit-verifier.yaml
@@ -143,7 +143,7 @@
     node: master
     days-to-keep: 90
     number-to-keep: 500
-    prune-dead-branches: false
+    prune-dead-branches: true
     scm:
       - gerrit:
           url: https://gerrit-review.googlesource.com/a/gerrit