Remove the `forked_project_links` table

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22226 we removed the ForkedProjectLink model from rails. Making sure this table isn't used anymore.

Don't forget to re-add the unique index index_forked_project_links_on_forked_to_project_id.

Next, we need to drop up the table as well but providing a way back: regenerating the data from the fork_network_members.

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The following discussion from gitlab-ce!22226 should be addressed:

• @reprazent started a discussion: (+8 comments)

  @gitlab-database Could you take a look at this migration?

  We could also decide to just leave the table for now but stop using it.