Remove always blank token attributes from data sources

This change removes deprecated token fields from GitLab webhook data sources. The token field was previously marked as deprecated because it would always return blank values for security reasons - tokens are only available when creating webhooks, not when reading their configuration later. The removal affects both group and project webhook data sources, cleaning up the code by eliminating unused fields from documentation, data models, and API response handling.

Fixes #6611 (closed)