fix(cli): remove vendored build.rs that references non-synced customer specs (#16300)

build.rs generates smoke-test constants from customer-specific OpenAPI
specs (e.g. src/bin/box/openapi.yaml) that are correctly excluded by the
sync manifest. Since it panics when those files don't exist, remove it
from the vendored copy. The cli-sdk manifest has been updated to
classify build.rs as not-synced going forward.

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: jsklan