[infra] Remove code for setting project_trigger_overrides.

The project_trigger_overrides field has been removed from the
chromium_tests properties since the luci_project field has been removed
and chromium_tests will now always trigger child builders in the same
project as the running builder.

Bug: 1306211
Change-Id: I64466e380e786331f3c2b79bfb4250fe265be0e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3533047
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#982430}