Remove GardeningServer.

This has been moved into the infra repo at:
https://chromium.googlesource.com/infra/infra/+/master/appengine/sheriff_o_matic/

NOTRY=true
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/728023004

git-svn-id: svn://svn.chromium.org/blink/trunk@185403 bbb929c8-8fbe-4397-9dbb-9b2b20218538