Remove deprecated features in 1.0.0

Before releasing the 1.0.0 version, we need to cleanup the code a bit.

One part of this is removing features or compatibility measures that are marked as "deprecated", e.g. compatibility imports for moved modules, and the `create_with_defaults()` method in the ValidataclassMixin.

NOTE: To avoid breaking changes, this needs to be done in the 1.0.0 release and not any earlier.