Remove Gerrit v2.13, plugins and Buck builds

Gerrit v2.13 is now officially unsupported and it is time
to remove the historical plugin builds and support for Buck
builds altogether.

Do not even query for changes that are in v2.13, so they do
not get a Verified -1 accidentally.

Change-Id: I8c13169b904e6e56057a85c7cc086c6d6203e876
88 files changed
tree: 23407e0a8e1995a6cd1e36d2970d469e2fddd403
  1. jenkins/
  2. jenkins-docker/
  3. worker/
  4. .gitignore