eolian: remove param @nonull

eolian: remove param @nonull

This has been deprecated for a while and is not strictly necessary
- as a part of an effort to stabilize Eolian, remove this. Eolian
will eventually gain support for versioning and use a reversed
behavior (i.e. no NULL by default), but the API it wlll use for
that will be very different. Features can always be added, it's
much harder to drop them.

@feature