Remove cores dimension from mac-angle-try.

This was preventing configs from finding new builders added that
had a different number of cores.

Bug: 822310
Change-Id: I59f452cb913a10cae8a54d8f0062f5fd2dc5cdff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562452Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649657}

infra/config/cr-buildbucket.cfg

@@ -499,7 +499,6 @@ builder_mixins {
   name: "mac-angle-try"
   mixins: "mac"
   mixins: "angle-try"
-  dimensions: "cores:4"
 }
 
 builder_mixins {