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

This reverts commit be5a2a38.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 553160 as the
culprit for failures in the build cycles as shown on:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2JlNWEyYTM4Yjg3N2NkNzk3YzJlNjJjMzcxOTQ2Nzg5Y2RkMTU0YzMM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.linux/Linux%20Builder%20%28dbg%29/123643

Sample Failed Step: compile

Original change's description:
> [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/1024729
> Reviewed-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}

Change-Id: I0c984c1aae350b8ab59954d6602b7a689ffa40f1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 731546
Reviewed-on: https://chromium-review.googlesource.com/1025284
Cr-Commit-Position: refs/heads/master@{#553195}

infra/config/global/cr-buildbucket.cfg

@@ -286,8 +286,6 @@ 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"
   }
@@ -296,8 +294,6 @@ 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"
   }