luci: switch 5 builders to builderless machine allocation.

Follow-up to crrev.com/i/1378754

Bug: 864819
Change-Id: I57937a8ee076d499a7e73d041d696ab61df71d56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654278
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668665}