[cr-buildbucket] Normalize chromium.swarm dimensions

Bug: 840631
TBR: tandrii
Change-Id: I85ea736f70f7241734d37dc0e6f897ca04575b5f
Reviewed-on: https://chromium-review.googlesource.com/1111022Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569434}

infra/config/global/cr-buildbucket.cfg

@@ -1749,7 +1749,7 @@ buckets {
     }
     builders {
       name: "Mac Swarm"
-      dimensions: "os:Mac-10.12.2"
+      dimensions: "os:Mac-10.13"
       mixins: "swarm-ci"
     }
     builders {
@@ -1775,7 +1775,6 @@ buckets {
     builders {
       name: "Linux Swarm"
       dimensions: "os:Ubuntu-14.04"
-      dimensions: "cores:16"
       mixins: "swarm-ci"
     }
     builders {