Fix wrong static method calls in UnsavedIndexConfiguration

I just looked at \Drupal\search_api\UnsavedIndexConfiguration and PhpStorm showed me four hard errors: For some reason, we redirect the static methods (preCreate() et al.) to calls on EntityInterface (so, e.g., EntityInterface::preCreate(…)) – which is, of course, nonsense.

Apparently, those methods are never used (which makes sense, as UnsavedIndexConfiguration isn’t actually an entity plugin class), but should probably still be fixed for the sake of less red squiggly lines.