python: Remove __all__ export from application.py

[svix-mman]

__all__ allows a from svix.api.application import *, but this was never a supported feature. And the __all__ is not in any of the other modules.

[svix-jplatte]

Not having __all__ does not mean that * imports don't work, it means that the * import imports every symbol from the module that's not _-prefixed. Still, makes sense to remove __all__.