[cr-buildbucket] Clear 'cores' dimension for ios-* builders.

ios-* builders may run on a mix of VMs with cores:8 and Mac Mini's with cores:4.

Bug: 731546
Change-Id: I056fbb5bd80ca50ef8005aa8bb99643853acd923
Reviewed-on: https://chromium-review.googlesource.com/1024729Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553160}

infra/config/global/cr-buildbucket.cfg

@@ -286,6 +286,8 @@ builder_mixins {
 builder_mixins {
   name: "ios-ci"
   mixins: "mac-ci"
+  # ios bots may run both on Mac Minis with cores:4 and VMs with cores:8.
+  dimensions: "cores:"
   recipe {
     name: "ios/unified_builder_tester"
   }
@@ -294,6 +296,8 @@ builder_mixins {
 builder_mixins {
   name: "ios-try"
   mixins: "mac-try"
+  # ios bots may run both on Mac Minis with cores:4 and VMs with cores:8.
+  dimensions: "cores:"
   recipe {
     name: "ios/try"
   }