Remove cores dimensions for clang

They need large core machines.
This is fine since auto_builder_dimensions is on.

Bug: 868213
TBR: tandrii
Change-Id: I3aa153e09afbd7e9ddc885441a09183f73727232
Reviewed-on: https://chromium-review.googlesource.com/1214220Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589683}

infra/config/global/cr-buildbucket.cfg

@@ -200,6 +200,7 @@ builder_mixins {
 
 builder_mixins {
   name: "clang-ci"
+  dimensions: "cores:"  # Mac uses 24 cores, Linux/Win uses 32 cores.
   recipe {
     properties: "mastername:chromium.clang"
   }