Don't build gitblit on Gerrit 2.13

GitBlit's Lucene version isn't compatible with the
one in Gerrit 2.13. Even if it builds correctly, it
would never work.

Having a binary available on the CI would be misleading
people in trying to use it.

Change-Id: I87778c648f6a01d3a71e0ae48441f26076e5c446
diff --git a/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml b/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
index 210c664..b01a37d 100644
--- a/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
@@ -85,7 +85,6 @@
       - 'plugin-gitblit-{branch}':
           targets: plugins/gitblit
           branch:
-            - stable-2.13
             - stable-2.12
             - stable-2.11
             - stable-2.10