Remove physical_clusters from config. (#2098)

We deprecated these clusters some time ago.

Author: fweikert

buildkite/instances.yml

@@ -61,10 +61,3 @@ instance_groups:
     service_account: [email protected]
     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