Revert "userdb: add birthDate field to JSON user records

This is a revert of a systemd PR that added a birthDate field to JSON user records in userdb. The revert cites privacy concerns, open source philosophy mismatch, practical enforcement issues, legal jurisdictional overreach, and lack of upstream consensus as reasons. The project may revisit if a zero-knowledge age-proof standard emerges or laws are clarified to exempt OSS projects.