src/chain_spec: remove code_substitutes hack

[melekes]

and amend the field's docs
Closes #1844

[melekes]

@tomaka is there anything else I've missed as a part of #1844?

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
       +1048 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::h2f7a75e8efdc0153
       -1048 ┊ hashbrown::raw::RawTable<T,A>::reserve_rehash::he09bab3e2f9e44fa
        -627 ┊ <smoldot::chain_spec::structs::HashHexString as serde::de::Deserialize>::deserialize::h76e6345c9cf6408d
        +627 ┊ <smoldot::chain_spec::structs::HashHexString as serde::de::Deserialize>::deserialize::ha47a755a4e2952b5
        +533 ┊ <serde_json::de::MapAccess<R> as serde::de::MapAccess>::next_key_seed::h741a4619a8338eb7
        -533 ┊ <serde_json::de::MapAccess<R> as serde::de::MapAccess>::next_key_seed::h79ad21ef3e59ff94
        +531 ┊ <&mut serde_json::de::Deserializer<R> as serde::de::Deserializer>::deserialize_map::h0c8fc846c211baff
        -531 ┊ <&mut serde_json::de::Deserializer<R> as serde::de::Deserializer>::deserialize_map::ha07102b503700973
        -435 ┊ <smoldot::chain_spec::structs::NumberAsString as serde::de::Deserialize>::deserialize::h3848028af9553240
        +330 ┊ <hashbrown::external_trait_impls::serde::map::<impl serde::de::Deserialize for hashbrown::map::HashMap<K,V,S>>::deserialize::MapVisitor<K,V,S> as serde::de::Visitor>::visit_map::h067849e95faf8e1c
        -330 ┊ <hashbrown::external_trait_impls::serde::map::<impl serde::de::Deserialize for hashbrown::map::HashMap<K,V,S>>::deserialize::MapVisitor<K,V,S> as serde::de::Visitor>::visit_map::he1d81450e6534661
        +288 ┊ hashbrown::raw::RawTable<T,A>::clone_from_impl::h951184c2030bb306
        -288 ┊ hashbrown::raw::RawTable<T,A>::clone_from_impl::he2d490875294438d
        +272 ┊ hashbrown::map::HashMap<K,V,S,A>::insert::h2c24603d776d7459
        -272 ┊ hashbrown::map::HashMap<K,V,S,A>::insert::h8b07cfff8b6df5dc
        +252 ┊ <serde_json::de::MapKey<R> as serde::de::Deserializer>::deserialize_u64::h66a68cd40a067522
        +210 ┊ smoldot::sync::all::AllSync<TRq,TSrc,TBl>::desired_requests::hb0b63a2e4c0d37fa
        -198 ┊ <hashbrown::raw::RawIterRange<T> as core::iter::traits::iterator::Iterator>::next::h7462dd76a8922aba
        +198 ┊ <hashbrown::raw::RawIterRange<T> as core::iter::traits::iterator::Iterator>::next::h7b96aee755dfffb5
        +172 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hcd76df886a0fe929
        -103 ┊ ... and 132 more.
       -2504 ┊ Σ [152 Total Rows]

[tomaka]

The block number is actually in the form of a string (probably to avoid precision issues with JavaScript)
You can find an example here: https://raw.githubusercontent.com/paritytech/polkadot/master/node/service/res/polkadot.json
I would appreciate a test to make sure that this parses correctly. You can probably just tweak the existing example.json.

I've removed this field from the polkadot.json spec in the smoldot repo, could you maybe put it back as part of this PR?