Cleanup proto rules and point to the source of truth

They have been moved to https://github.com/protocolbuffers/protobuf

Not sure who would be the best person to reach out about the status of wether the proto rules outside rules_python are useable already.

Start with the issue context and the protocolbuffers/protobuf repository linked in the report. Determine which proto rules remain in rules_python and whether the replacement rules are usable. The work is complete only once the obsolete rules are cleaned up and the source of truth is clearly pointed to, but the intended scope needs confirmation.

Vom Indexierungsmodell aus dem Issue-Text verfasst.