Remove root_type_id from TypeInfo::Custom.

[tritao]

Description

This PR removes root_type_id from TypeInfo::Custom.

This was used for resolving Self in associated types, but it's not needed, we can resolve those using the regular self_type we pass to name resolution.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6676 will not alter performance

_{Comparing tritao:associated-types-unify (edeed2c) with master (4efd248)}

Summary

✅ 22 untouched benchmarks

[IGI-111]

Always happy to merge delta negative PRs! 👍