Build the batch plugin on its own stable-2.15 branch

Change-Id: Ibd48a39d8b7d6695db3828314e3cf8f486abe893
diff --git a/jenkins/gerrit-bazel-plugin-batch.yaml b/jenkins/gerrit-bazel-plugin-batch.yaml
index 4b81a99..71d3f01 100644
--- a/jenkins/gerrit-bazel-plugin-batch.yaml
+++ b/jenkins/gerrit-bazel-plugin-batch.yaml
@@ -1,8 +1,7 @@
 - project:
     name: batch
     jobs:
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+      - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
-          gerrit-branch:
             - stable-2.15