Remove incorrect av/__init__.pyi

[jlaine]

We do not need this type stub, since __init__.py is already a Python file. What's more the file is incorrect since it uses star imports, which the actual __init__.py does not.

[WyattBlue]

I'm glad that __init__.pyi can be removed. However, that does mean we need to write a stub for audio/fifo since mypy is checking __init__.py now. I actually went and already did that and applied your changes #1433, so we don't need this pull request now.