Remove "cores" dimensions for Win/Linux VMs on chromium.gpu.fyi.

We want to be able to use smaller VMs for these tester-only machines,
and currently they're defaulting to requiring 8 cores.

This will be followed by a CL that removes this requirement for the
same VMs on the chromium.gpu waterfall.

Bug: 881625
Bug: 948220
No-Try: True
Change-Id: Iaf5400bfe7319be37a045b0cc781864eb6755f42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548545
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646588}

infra/config/cr-buildbucket.cfg

@@ -440,6 +440,12 @@ builder_mixins {
   mixins: "linux"
   mixins: "gpu-fyi-ci"
   mixins: "gpu-slow-bot"
+  # Remove the cores dimensions for this mixin so that VMs of differing sizes
+  # can be used.
+  # This works here only because we use auto_builder_dimensions,
+  # so that only the bots that were earmarked for the builders
+  # will be allowed to use them anyways.
+  dimensions: "cores:"
 }
 
 builder_mixins {
@@ -611,6 +617,12 @@ builder_mixins {
   mixins: "win"
   mixins: "gpu-fyi-ci"
   mixins: "gpu-slow-bot"
+  # Remove the cores dimensions for this mixin so that VMs of differing sizes
+  # can be used.
+  # This works here only because we use auto_builder_dimensions,
+  # so that only the bots that were earmarked for the builders
+  # will be allowed to use them anyways.
+  dimensions: "cores:"
 }
 
 builder_mixins {