Remove unused add_member flow method

What does this MR do?

Remove un-used add_member method that was accidentally re-introduced in !50701 (merged)

See !50701 (comment 514753556)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content