Correctly remove the core count dimension from mac libfuzz luci bot.

R=jbudorick, martiniss

Bug: 790372
Change-Id: I56f5374182eaa3779459c05203ebb5cb1d94802f
Reviewed-on: https://chromium-review.googlesource.com/c/1359030Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613177}

infra/config/global/cr-buildbucket.cfg

@@ -2017,6 +2017,11 @@ buckets {
     builders {
       name: "Libfuzzer Upload Mac ASan"
       dimensions: "os:Mac-10.13"
+      # Remove core count from the requested dimensions since the builder's
+      # pool has a mix of Mac models currently.
+      # TODO(crbug.com/790372): Remove this when the builder has a single Mac
+      # model in its pool.
+      dimensions: "cores:"
       mixins: "fyi-ci"
       # mixins: "fuzz-ci"
       mixins: "libfuzzer"