message: cluster: remove unused proto import
  This warms when building:
  > envoy/api/v2/cluster/filter.proto:12:1: warning: Import google/protobuf/struct.proto but not used.
diff: @@ -9,7 +9,6 @@ option csharp_namespace = "Envoy.Api.V2.ClusterNS";
  option ruby_package = "Envoy.Api.V2.ClusterNS";
  import "google/protobuf/any.proto";
  -import "google/protobuf/struct.proto";
  import "validate/validate.proto";
  import "gogoproto/gogo.proto";
  