Stop building plugin-manager for Gerrit >= v3.0

plugin-manager is a core-plugin from v3.0 onwards.
It does not need to be built anymore as a separate project.

Builds for any version <= v2.16 will stay on Gerrit-CI.

Change-Id: If2194f2536f362da4faf31957163c5b8e0ee5fad
diff --git a/jenkins/gerrit-buck-bazel-plugin-plugin-manager.yaml b/jenkins/gerrit-buck-bazel-plugin-plugin-manager.yaml
index 5e46c05..d8cf449 100644
--- a/jenkins/gerrit-buck-bazel-plugin-plugin-manager.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-plugin-manager.yaml
@@ -10,9 +10,3 @@
             - stable-2.16
             - stable-2.15
             - stable-2.14
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
-          branch:
-            - master
-          gerrit-branch:
-            - master
-            - stable-3.0