Archive the Generic Associated Types Initiative

[lcnr]

With the stabilization of GAT this initiative is no longer active. While there are currently still limitations on GATs we'll lift in the future, these are part of larger features, e.g. implied bounds on binders. This initiative itself has been inactive since then.

cc @nikomatsakis @jackh726

[github-actions]

Dry-run check results

[WARN  sync_team] sync-team is running in dry mode, no changes will be applied.
[INFO  sync_team] synchronizing crates-io
[INFO  sync_team] synchronizing github
[INFO  sync_team] 💻 Team Diffs:
    ❌ Deleting team 'rust-lang/initiative-generic-associated-types'
    💻 Repo Diffs:
    📝 Editing repo 'rust-lang/bors':
      Rulesets:
          Ruleset for main
            No changes
    📝 Editing repo 'rust-lang/generic-associated-types-initiative':
      Archive
      Permission Changes:
        Removing team 'initiative-generic-associated-types''s write permission