Remove __all__ use in PICMI and PyPIConGPU

[BrianMarre]

PICMI and PyPIConGPU __init__.py files specify an __all__ variable that is supposed to overwrite the default behavior of from picongpu.picmi import *, but do not actually diverge from the default behavior.

We should therefore not specify them at all.

[BrianMarre]

@chillenzer pre-commit removes all import statements from __init__.py if we do not include an __all__ is this intended behaviour?

[chillenzer]

Of course, it's not pre-commit immediately but ruff. These are the pertinent pages: https://docs.astral.sh/ruff/rules/unused-import/ and https://docs.astral.sh/ruff/settings/#lint_ignore-init-module-imports

As the latter is already deprecated, you can decide for yourself if adding a redundant import ... as ..., a redundant __all__ or #noqa unused-import (on the file level, of course) is the least bitter pill. I would personally say the former is still slightly easier to maintain but not by as large of a margin as before.