Remove custom fields from entity_keys in entity definition

Problem/Motivation

As we just understood the purpose of entity_keys, we should probably remove custom values that are not core entity keys.

See https://drupal.stackexchange.com/questions/194316/what-is-the-purpose-of...

TL;DR: Entity keys are mappings of core entity keys, like for example

nid => id (Node)
tid => id (Taxonomy term)
...

we added some custom values there, that are probably not needed. It doesn't cause any harm, as it seems, for now. But if it's wrong, it should be removed. Furthermore, these fields may never be NULL!

Before wo change this, please compare to other custom modules and ensure it's right.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes