Remove possibility to record "overige" PartijIdentificatoren

[swrichards]

In the current API spec it is not effectively possible to record "overige" (i.e. non-formal, such as bsn or kvk) identificatoren. This is for two reasons:

1. The various enums for register, object type, etc. contain an 'overig' option, but this is incomplete: you have no good place to specify what kind of overig object it is (except possibly in the anderePartijIdentificator field, but that does not seem very consistent)
2. Now that the various meta-attributes are unique together, you can also only add a single "overig" identifier (for a given identifier value).

This means that, currently, "overige" identificatoren are broken. We need a better solution for this, but we also do not want to delay the release of 2.6.0 for this. The proposal here is therefore to:

1. Remove 'overig' from all the identificator enums
2. Remove the field anderePartijIdentificator
3. Make partijIdentificator non-nullable in the serializer

So that the current object is at least coherent and consistent for the "formal" registrations. We will then re-introduce a mechanism for overige identificatoren after we agree on a proposal with the various stakeholders. This is preferable to a release in which overige identificatoren are effectively broken.

Current API spec: