Revert "cr-buildbucket-dev: use auto_builder_dimension for luci.chromium.ci."

This reverts commit 99255887.

Reason for revert: added more machines, going to try out warm caching

Original change's description:
> cr-buildbucket-dev: use auto_builder_dimension for luci.chromium.ci.
> 
> Bug: 868525
> Change-Id: I65aabb60d4823423d7f0bd4d5b51ea145c665658
> Reviewed-on: https://chromium-review.googlesource.com/1176471
> Reviewed-by: Nodir Turakulov <nodir@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583450}

TBR=nodir@chromium.org,jbudorick@chromium.org

Change-Id: If807d179cf51089c8332a627ae4d2de78468e79a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 868525
Reviewed-on: https://chromium-review.googlesource.com/1178993Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583856}

infra/config/global/cr-buildbucket-dev.cfg

@@ -51,7 +51,6 @@ buckets {
       execution_timeout_secs: 10800  # 3h
       service_account: "chromium-ci-builder-dev@chops-service-accounts.iam.gserviceaccount.com"
       build_numbers: YES
-      auto_builder_dimension: YES
       recipe {
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"