Don't save ImmutableConfig to class property in WikiNodeTalkLocalTaskController

We're saving ImmutableConfig to a class property, which is bad practice:

https://github.com/neurocracy/drupal-omnipedia-discourse/blob/c034b2df15f3ea053e4beedcdcbdb0a5ac3b63be/src/Controller/WikiNodeTalkLocalTaskController.php#L44

See Dependency injection anti-patterns in Drupal by Matt Glaman.