Stop building the plugin-manager plugin on the master branch

The plugin-manager plugin is now a core plugin, so we don't need a
separate build.

Feature: Issue 10717
Change-Id: Idcf7e2bd072520bc7194d94b7271d2eb2fc6458c
tree: 3cba33819560c5ece3b611e46e0995c49cdd431b
  1. .gitignore
  2. jenkins-docker/
  3. jenkins/
  4. worker/