Make buildbotless trybots only support Gerrit

BUG=skia:5723

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4460

Change-Id: Ia5630de72f8b473551a5357fdf50eb3dd288b9a7
Reviewed-on: https://skia-review.googlesource.com/4460
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
3 files changed