Do not register StepAction for conversion

`StepAction` only exists in `v1alpha1` and thus doesn't need to be
registered in the conversion webhook.

Prior to this commit, the webhook errors out (several times) with the
following:

```
custom resource \"stepactions.tekton.dev\" isn't configured for webhook conversion
```

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

cmd/webhook/main.go

@@ -153,7 +153,6 @@ func newConfigValidationController(name string) func(context.Context, configmap.
 
 func newConversionController(ctx context.Context, cmw configmap.Watcher) *controller.Impl {
 	var (
-		v1alpha1GroupVersion           = v1alpha1.SchemeGroupVersion.Version
 		v1beta1GroupVersion            = v1beta1.SchemeGroupVersion.Version
 		v1GroupVersion                 = v1.SchemeGroupVersion.Version
 		resolutionv1alpha1GroupVersion = resolutionv1alpha1.SchemeGroupVersion.Version
@@ -171,10 +170,6 @@ func newConversionController(ctx context.Context, cmw configmap.Watcher) *contro
 		// conversions to and from all types.
 		// "Zygotes" are the supported versions.
 		map[schema.GroupKind]conversion.GroupKindConversion{
-			v1alpha1.Kind("StepAction"): {
-				DefinitionName: pipeline.StepActionResource.String(),
-				HubVersion:     v1alpha1GroupVersion,
-			},
 			v1.Kind("Task"): {
 				DefinitionName: pipeline.TaskResource.String(),
 				HubVersion:     v1beta1GroupVersion,