Remove redundant field in service.rs

[tomaka]

Before #2264, we were maintaining an additional redundant list of connections in service.rs in order to avoid having to lock a mutex.
After the big refactor, this redundant list is no longer needed as we can directly access the list in the underlying data structure.

[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
─────────────┼─────────────────────────────────────────────────────────────────────────────────────────────────
      -10473 ┊ smoldot::network::service::ChainNetwork<TNow>::next_event::h8c3fbc17c9efa90a
      +10386 ┊ smoldot::network::service::ChainNetwork<TNow>::next_event::hd1aff972b8b32faf
       -1274 ┊ smoldot::network::service::ChainNetwork<TNow>::new::h1194499a5e459cbf
       +1211 ┊ smoldot::network::service::ChainNetwork<TNow>::new::h39de0984c66b61ed
        -928 ┊ smoldot::network::service::ChainNetwork<TNow>::next_start_connect::h55983c9a6cec2033
        +928 ┊ smoldot::network::service::ChainNetwork<TNow>::next_start_connect::hcfb51e434abb3f14
        -860 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h4fae1d452d8919d3
        +860 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h94c6580e84bc2764
        +721 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h08a464fb88d3f722
        -721 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h9ec50706633ae426
        +698 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h003dd42f7ea8a06a
        -698 ┊ smoldot::network::service::ChainNetwork<TNow>::discover::h473c4c6211b9f198
        -588 ┊ smoldot::network::service::ChainNetwork<TNow>::respond_identify::hb6f86ca10a3d59d7
        +588 ┊ smoldot::network::service::ChainNetwork<TNow>::respond_identify::hbf285c3d3cacdedd
        -547 ┊ smoldot::network::service::ChainNetwork<TNow>::pending_outcome_err::h8354388b734ab859
        +543 ┊ smoldot::network::service::ChainNetwork<TNow>::pending_outcome_err::h60cac5a8ebddecde
        +521 ┊ smoldot::network::service::ChainNetwork<TNow>::start_kademlia_discovery_round::ha6970526e10c4c98
        -521 ┊ smoldot::network::service::ChainNetwork<TNow>::start_kademlia_discovery_round::he9e4d0fc443e8deb
        +448 ┊ smoldot::network::service::ChainNetwork<TNow>::start_blocks_request_inner::h4348c053d975dd9e
        -448 ┊ smoldot::network::service::ChainNetwork<TNow>::start_blocks_request_inner::hbca2a368f08ce85d
         -12 ┊ ... and 55 more.
        -166 ┊ Σ [75 Total Rows]

[melekes]

👍