refactor: Deprecated user config from IConfig correctly

[CarlSchwan]

Mark the methods in the interface deprecated instead of just the one in the implementation.

Summary

Mark the methods in the interface deprecated instead of just the one in the implementation.

One of them was not on the public interface, so I directly removed it and ported the only usage.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)
• [x] Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• [x] Milestone added for target branch/version (ex: 32.x for stable32)