Build webhooks plugin separately for master/2.14/2.15

Change-Id: Ia50e92720ea6a3b351425190445a6f0f74b6a22f
diff --git a/jenkins/gerrit-bazel-plugin-webhooks.yaml b/jenkins/gerrit-bazel-plugin-webhooks.yaml
index b010535..ea93df9 100644
--- a/jenkins/gerrit-bazel-plugin-webhooks.yaml
+++ b/jenkins/gerrit-bazel-plugin-webhooks.yaml
@@ -3,11 +3,8 @@
     jobs:
       - 'plugin-{name}-bazel-{branch}':
           branch:
-            - stable-2.13
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
-          branch:
             - master
-          gerrit-branch:
-            - master
+            - stable-2.15
             - stable-2.14
+            - stable-2.13