[cr-buildbucket.cfg] Remove unneeded dimensions

The device builders weren't getting picked up because of the extra cores:8
and cpu:x84-64 dimensions.

Bug: 868216
Change-Id: If098ecaadc3840a232bafc811c4405c5ed29585f
diff --git a/tools/luci/config/cr-buildbucket.cfg b/tools/luci/config/cr-buildbucket.cfg
index 29cca39..8df8433 100644
--- a/tools/luci/config/cr-buildbucket.cfg
+++ b/tools/luci/config/cr-buildbucket.cfg
@@ -27,8 +27,6 @@
   swarming {
     hostname: "chromium-swarm.appspot.com"
     builder_defaults {
-      dimensions: "cores:8"
-      dimensions: "cpu:x86-64"
       dimensions: "pool:luci.art.ci"
       service_account: "art-ci-builder@chops-service-accounts.iam.gserviceaccount.com"
       execution_timeout_secs: 10800  # 3h