Remove deprecated `#forked_from?` check

What does this MR do?

I forgot to clean up this bit in we switched the forked_from_project relation to use fork networks in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22226

@dzaporozhets Caught this in https://gitlab.com/gitlab-org/gitlab-ce/commit/70716a1292ca5910908ba37a9d113c8b5a221bb7#note_185977691 🙏

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content