Remove 'Enum' Endpoints

Enum endpoints were introduced to make it easy to see possible (user-supplied) values. But I do not think this is useful anymore. For fields that have restricted values, we employ Taxonomies now, which guarantee that user values adhere to certain terms. On the other hand, if there is no such restriction, then this enum doesn't make much sense. It is merely a (premature) optimization to normalize string data. We will keep that under the hood (in the interest of prioritizing other things) but no longer expose this complexity to users.