Remove the temporary `Published*` aliases from `models_importstab.py`

[candleindark]

Remove these two aliases from dandischema/models_importstab.py as part of the #419 follow-up that drops the deprecated aliases from dandischema/models.py:

PublishedDandiset = Dandiset
PublishedAsset = Asset

The stub carries them only to stay a drop-in replacement for that module, so they should go as soon as it no longer needs to be.

Why the aliases exist

dandischema/models_importstab.py on linkml-conversion is what the conversion pipeline installs as dandischema/models.py on linkml-auto-converted. The aliases mirror the deprecated ones #419 left in dandischema/models.py, and they are needed because metadata.SCHEMA_MAP still names PublishedDandiset and PublishedAsset while publish_model_schemata resolves each key with getattr(models, class_). Without them the pydantic2json stage of ./tools/linkml_conversion fails:

AttributeError: module 'dandischema.models' has no attribute 'PublishedDandiset'

They were added in 54138ad on linkml-conversion and confirmed working by the regeneration at 0ce51b3 on linkml-auto-converted, where models_pydantic/published-dandiset.json and models_pydantic/published-asset.json resolve through the aliases and come out byte-identical to dandiset.json and asset.json.