Remove ForkedProjectLink model

What does this MR do?

This deletes the deperecated ForkedProjectLink model, which has been replaced by ForkNetwork and ForkNetworkMember in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14422. All remaining relations using this model have been re-written to use the new models.

I thought that we could not drop the forked_project_links table, as there were some places in the API not using the new datamodel yet. But rescheduling the data migration was also taken care of in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15709. Thanks Fran!

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/38883

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary: Not needed, developer change
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides