Address Untested to_json Extension Module Code
There's an old TODO note about untested code in the extension module hanging around in objToJSON.c:

https://github.com/pandas-dev/pandas/blob/bfff080275b4456b28d71f0c7b4ec9e678d4270c/pandas/_libs/src/ujson/python/objToJSON.c#L504

Interestingly enough, this is actually tested by the `np.datetime64` parameter in `test_encode_as_null` as part of the `test_ujson.py` module:

https://github.com/pandas-dev/pandas/blob/bfff080275b4456b28d71f0c7b4ec9e678d4270c/pandas/tests/io/json/test_ujson.py#L388

The intent here is somewhat blurry. I think the `test_ujson.py` module was created when it was up for discussion if we wanted the JSON serializers to be publicly exposed at the top level (see #9147) which I don't believe is still something we want to do. At that time it would make sense to explicitly support the `np.datetime64` type.

The remaining question then is whether `np.datetime64` should be represented within a pandas container. It seems that this can be held in a Series with an object dtype but not a DataFrame, as shown below:

```python
>>> ser = pd.Series([np.datetime64("2000-01-01")], dtype=object)
>>> type(ser.iloc[0])
numpy.datetime64
>>> type(ser.to_frame().iloc[0, 0])
pandas._libs.tslibs.timestamps.Timestamp
```

If we don't want `np.datetime64` objects to be contained then we can delete this code altogether. If we do we should add test coverage for it, but then also address the inconsistency across container types above