internal: remove unused Labels on Iteration

The iteration structure that we use for linking a packer build to an
iteration on HCP defines a `Labels' attribute, which is never set nor
read from at any point.

Since it is unused, we remove it in this commit.

internal/hcp/registry/types.iterations.go

@@ -16,7 +16,6 @@ type Iteration struct {
 	AncestorSlug   string
 	Fingerprint    string
 	RunUUID        string
-	Labels         map[string]string
 	builds         sync.Map
 	expectedBuilds []string
 }