test: remove RetroactiveDefaultStorageClass feature gate

Since the feature is GA and locked to true, tests can no longer set it
to false. Cleaning up by removing all references to this feature gate
from tests.

Feature gate will be removed in v1.29.

pkg/controller/volume/persistentvolume/pv_controller_test.go

@@ -754,8 +754,6 @@ func TestModifyDeletionFinalizers(t *testing.T) {
 }
 
 func TestRetroactiveStorageClassAssignment(t *testing.T) {
-	// Enable RetroactiveDefaultStorageClass feature gate.
-	defer featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.RetroactiveDefaultStorageClass, true)()
 	tests := []struct {
 		storageClasses []*storagev1.StorageClass
 		tests          []controllerTest