Remove the deprecated `PublishedDandiset` / `PublishedAsset` aliases

Follow-up from #419, which consolidated `PublishedDandiset` into `Dandiset` and `PublishedAsset` into `Asset` but kept the old names in `dandischema/models.py` as deprecated aliases so that consumers would keep importing successfully:

```python
PublishedDandiset = Dandiset
PublishedAsset = Asset
```

Getting rid of them takes, in order:

- [ ] Remove the use of the aliases in the consumers, e.g. dandi-archive.
- [ ] Remove the aliases from `dandischema/models.py`, together with the remaining in-repo uses in `to_datacite` and the tests.
- [ ] Decide what becomes of the `PublishedDandiset` / `PublishedAsset` keys of `SCHEMA_MAP`, since `publish_model_schemata` resolves each key with `getattr(models, class_)`.
- [ ] Remove the copies in `models_importstab.py` on the `linkml-conversion` branch (tracked separately in #439).

Findings as of filing (recheck before implementing)

**dandi-cli is clear.** Verified at `e982b3bc`: it uses `BareAsset`, which stays, but neither deprecated alias.

**dandi-archive uses both, in four places.** `api/views/schema.py` and `api/tests/test_schema.py` are the two #419 called out: each builds its mapping from `model.__name__`, so both already collapse from four entries to two silently rather than failing.

The other two are load-bearing and fail at call time rather than at import, because they pass the name as a string:

- `dandiapi/api/services/publish/__init__.py:196` — `validate(..., schema_key='PublishedDandiset', json_validation=True)`
- `dandiapi/api/services/metadata/__init__.py:65,151` — the same with `'PublishedAsset'` and `'PublishedDandiset'`

`validate` resolves the key with `getattr(models, schema_key)` (`dandischema/metadata.py:234` and `:296`), and both call sites sit on the publish path, so the breakage would surface at publication rather than at startup.

**The `SCHEMA_MAP` question.** Since #419 the `published-*.json` files have been byte-identical to `dandiset.json` and `asset.json`, so emitting them buys nothing, and the `linkml-conversion` branch has already stopped generating them on the LinkML side (`7ed984e9`). But `SCHEMA_MAP` is also the accepted `schema_key` vocabulary, and `_get_jsonschema_validator` builds a URL into a released schema version from it, so dropping the keys outright would also drop the ability to validate older published metadata under its historical key.

Start with dandischema/models.py, the in-repo uses in to_datacite and tests, and SCHEMA_MAP; then inspect the listed dandi-archive call sites and dandischema/metadata.py validation lookups. Remove deprecated alias usage without breaking publication validation, decide how historical SCHEMA_MAP keys should behave, and update the affected tests and consumers.

Written by the indexing model from the issue text.