Remove physical_clusters from config. (#2098)

We deprecated these clusters some time ago.
diff --git a/buildkite/instances.yml b/buildkite/instances.yml
index db33032..d4ff498 100644
--- a/buildkite/instances.yml
+++ b/buildkite/instances.yml
@@ -61,10 +61,3 @@
     service_account: buildkite-trusted@bazel-public.iam.gserviceaccount.com
     image_family: bk-windows
     metadata_from_file: windows-startup-script-ps1=startup-windows-pdssd.ps1
-physical_clusters:
-  - name: buildkite-imacpro
-    count: 20
-  - name: bk-testing-macpro
-    count: 4
-  - name: bk-trusted-macpro
-    count: 4