source_tree_test_config: remove unused only_test_build_target field

This has been replaced with cq_only_test_builders

BUG=chromium:1062039
TEST=generate

Change-Id: I47a11061ad83c4e4b8b7c508cec14344f12564bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2110912
Tested-by: Sean Abraham <seanabraham@chromium.org>
Auto-Submit: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
Commit-Queue: Dhanya Ganesh <dhanyaganesh@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
2 files changed