[reclient] Cleanup chromeos shadow builders.

Bug: b/231326367
Change-Id: I36dad5466a178844b358d765506fea5c804c4717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3646173
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Commit-Queue: Ramy Medhat <abdelaal@google.com>
Cr-Commit-Position: refs/heads/main@{#1002862}