No longer discard justifications of blocks that were already known

[tomaka]

Fixes a TODO in the code.

This TODO was from a time when we were handling justifications very naively by attaching them to blocks.
After #2400, we now properly handle justifications by attaching them to a (block, source) tuple, meaning that the problem of replacing existing justifications with other ones doesn't exist anymore.

This TODO should have been fixed in #2400, but I forgot.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────
        +754 ┊ smoldot::sync::all_forks::AddBlockOccupied<TBl,TRq,TSrc>::replace::he158aecb8287a97b
        -634 ┊ smoldot::sync::all_forks::AddBlockOccupied<TBl,TRq,TSrc>::replace::hbf3503201a3e9780
         +93 ┊ smoldot::sync::all_forks::FinishAncestrySearch<TBl,TRq,TSrc>::add_block::h9adac7250d79add4
         +42 ┊ smoldot::sync::all_forks::AddBlockOccupied<TBl,TRq,TSrc>::cancel::h5cac5d15d480d406
         -24 ┊ smoldot::sync::all::AllSync<TRq,TSrc,TBl>::blocks_request_response::h7d78c4f7a1206829
         +16 ┊ data[0]
        +346 ┊ Σ [6 Total Rows]