[code coverage] Do not set builder dimension on linux-coverage-rel.

This is because we don't want to allocate dedicated hardware for this
cq builder yet, as it's optional and still under development.

R=stgao,smut,liaoyuke

Change-Id: Ie0d896993ed54172c502af938229612e9e268714
Reviewed-on: https://chromium-review.googlesource.com/c/1325073
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Reviewed-by: smut <smut@google.com>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606260}

infra/config/global/cr-buildbucket.cfg

@@ -3059,6 +3059,7 @@ buckets {
       mixins: "linux-try"
       mixins: "goma-j150"
       name: "linux-coverage-rel"
+      auto_builder_dimension: NO
     }
     builders {
       mixins: "linux-try"