drop old refs/tryjobs/ namespace

This was used by old cros tryjob tooling.

Change-Id: I6f3409a5fc406453caeae788d9d13c8f439fdd28
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/+/6443758
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>