source_tree_test_config: remove deprecated cq_only_test_builders field

This has been fully replaced with cq_only_test_groups.

Must be submitted after https://crrev.com/i/2818665

BUG=chromium:1012480,chromium:1062039
TEST=regen

Change-Id: Ia49b043f40268a0637ac96da93b650345ca4927a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2124827
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Commit-Queue: Sean Abraham <seanabraham@chromium.org>
Tested-by: Sean Abraham <seanabraham@chromium.org>
2 files changed