Remove unused edisgo/tools/powermodels_io.py

[jldonner]

Description

Remove the unused legacy PowerModels converter from
edisgo/tools/powermodels_io.py.

The active OPF converter remains in
edisgo/io/powermodels_io.py.

The legacy converter is not imported by the code base and contains
conversion logic and storage-efficiency values that differ from the
active OPF implementation. Keeping it makes investigations of the OPF
pipeline unnecessarily confusing.

Type of change

• Deleted edisgo/tools/powermodels_io.py
• Verified that no references to edisgo.tools.powermodels_io remain

Closes #699

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file