gguf-py : avoid requiring pyside6 for other scripts (#13036)

- gguf-py : remove gguf-py/gguf/scripts/__init__.py because it's not needed

Implicit namespaces are supported since Python 3.3 (https://peps.python.org/pep-0420/),
and the entrypoints in pyproject.toml can directly refer to the main functions.