testplan: Remove references to deprecated proto fields.

- Fields removed in
https://chromium-review.googlesource.com/c/chromiumos/config/+/3209009,
see that CL for explanation for removal.

- Future CLs will evalute Starlark files to generate HWTestPlans. For
now, just leave skeleton of Generate method.

BUG=b:182898188
TEST=CQ
TEST=`emerge test-plan`

Change-Id: I631e2c9df74d6ef0f1e51853d6967eeb9d7cdb2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3217811
Reviewed-by: Tim Bain <tbain@google.com>
Commit-Queue: Andrew Lamb <andrewlamb@chromium.org>
Tested-by: Andrew Lamb <andrewlamb@chromium.org>
10 files changed