Nodes do not implement `__delattr__`

```python
import roman_datamodels.datamodels as rdm, roman_datamodels.maker_utils as mk
m = mk.mk_datamodel(rdm.ImageModel)
del m.meta.ref_file.dark
```

Produces:
```
AttributeError: 'RefFile' object has no attribute 'dark'
```

Expected that "dark" will be deleted from "ref_file". A workaround is to use `__delitem__` (which is implemented):
```
del m.meta.ref_file["dark"]
```