Remove __all__ to discourage wildcard imports

[janosh]

I think wildcard imports are a bad practice and a feature I wish Python didn't support.

By defining __all__ across atomate2 modules, we're encouraging this practice. It's additional maintenance burden to keep the list of strings in sync with the actual symbol names in a module. It would also increase ecosystem consistency to remove them since other MP repos do not use __all__.

[chiang-yuan]

Fewer lines are always better 😎