GroupRelationshipStorage::createForEntityInGroup() needlessly wraps config entity

Problem/Motivation

Not really a bug, but GroupRelationshipStorage::createForEntityInGroup() has:

    // If the entity is config, we need to use the wrapper for it.
    if ($group_relation_type->handlesConfigEntityType()) {
      $storage = $this->entityTypeManager->getStorage('group_config_wrapper');
      assert($storage instanceof ConfigWrapperStorageInterface);
      $entity = $storage->wrapEntity($entity);
    }

However, when we further down create the GroupRelationship and assign the target entity, it will always get wrapped regardless because of RelationshipEntityReferenceItem::setValue(). So let's remove the extra call to wrapEntity() and make the storage code easier to read.

Data model changes