AsdfFile does not implement __delitem__

[braingram]

Description of the problem

AsdfFile implements __setitem__ and __getitem__ for easy tree access but does not implement __delitem__

Example of the problem

> import asdf
> af = asdf.AsdfFile({'foo': 1})
> del af["foo"]
AttributeError: __delitem__

System information

asdf version: main
python version: 3.10
operating system: mac