Don't remove source knows block after verification

[tomaka]

Fixes a bug introduced in #2114

I don't know why I had added this call to remove_sources_known_block after a block has been successfully verified. Not only it is obviously wrong, but in #2114 I tried to not change the behavior and added this call that explicitly changes the behavior.

Unfortunately the lack of testing didn't help, but the problem got caught by https://github.com/paritytech/substrate-connect/runs/5590927994?check_suite_focus=true

[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.

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 Delta Bytes │ Item
─────────────┼─────────────────────────────────────────────────────────────────────────────────
         -16 ┊ smoldot::sync::all_forks::HeaderVerify<TBl,TRq,TSrc>::perform::h52cc25e098199667
         -16 ┊ Σ [1 Total Rows]