python: remove package-specific overrides for .repo-metadata.json

[jskeet]

In order to minimize the diff for Python, PythonPackage has fields to override the values in .repo-metadata.json. These are in PythonPackage rather than Library as we hope they'll be temporary. Once the migration has been completed, we can remove them, check the diff, and either adjust sdk.yaml or live with the changes.

Note that although the Librarian code only changes how .repo-metadata.json is written, this then affects documentation - which is why it's safer to minimize the diff first and then check for changes after migration.

[jskeet]

Moving to post-migration stabilization.