apis: drop check for volumes with user namespaces

The second phase of user namespaces support was related to supporting
only stateless pods.  Since the changes were accepted for the KEP, now
the scope is extended to support stateful pods as well.  Remove the
check that blocks creating PODs with volumes when using user namespaces.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/apis/core/validation/validation.go

@@ -3256,25 +3256,6 @@ func validateHostUsers(spec *core.PodSpec, fldPath *field.Path) field.ErrorList
 		return allErrs
 	}
 
-	// For now only these volumes are supported:
-	// - configmap
-	// - secret
-	// - downwardAPI
-	// - emptyDir
-	// - projected
-	// So reject anything else.
-	for i, vol := range spec.Volumes {
-		switch {
-		case vol.EmptyDir != nil:
-		case vol.Secret != nil:
-		case vol.DownwardAPI != nil:
-		case vol.ConfigMap != nil:
-		case vol.Projected != nil:
-		default:
-			allErrs = append(allErrs, field.Forbidden(fldPath.Child("volumes").Index(i), "volume type not supported when `pod.Spec.HostUsers` is false"))
-		}
-	}
-
 	// We decided to restrict the usage of userns with other host namespaces:
 	// 	https://github.com/kubernetes/kubernetes/pull/111090#discussion_r935994282
 	// The tl;dr is: you can easily run into permission issues that seem unexpected, we don't