Build avatars-external plugin on stable-2.15

Change-Id: Ia723f0cf933164f5a66a89ab8e7931de06dc3911
diff --git a/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml b/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
index 55c8a1e..73989bc 100644
--- a/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-avatars-external.yaml
@@ -4,10 +4,8 @@
       - 'plugin-{name}-{branch}':
           branch:
             - stable-2.13
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+      - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
-          gerrit-branch:
-            - master
             - stable-2.15
             - stable-2.14