Remove unused fallback protocol names system

[tomaka]

Ages ago, I added a dummy field called fallback_protocol_names that lets you specify a list of alternative protocol names that would be negotiated if the main one isn't supported by the remote.

Substrate has a similar feature, and it's very useful in order to do protocol upgrades. (e.g. #2116)

Unfortunately this has never been implemented.
Instead of leaving this dummy feature here, this PR just removes it. It can always be added back properly in the future.

[mergify]

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

[tomaka]

It turns out that this was even mis-implemented.
If a fallback name was passed, we would accept incoming substreams of that protocol then panic.

[github-actions]

twiggy diff report

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

---

 Delta Bytes │ Item
─────────────┼───────────────────────────────────────────────────────────────────────────────────────────────────────────────────
        -745 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::hf7a78941e21d4721
        +629 ┊ <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::next::hb1aceafb03d371e4
        -504 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::h2d83bbebd61f304a
        +504 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::hd2def653fc5270b5
        -435 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::h047d572d6d2d7f79
        +435 ┊ <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold::hef1b800efde7d999
        -419 ┊ core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once::hebadfbe7f6ac75f9
        -413 ┊ <core::iter::adapters::flatten::FlatMap<I,U,F> as core::iter::traits::iterator::Iterator>::next::h0a6214ba26d0de32
        +377 ┊ <core::iter::adapters::flatten::FlatMap<I,U,F> as core::iter::traits::iterator::Iterator>::next::h21a144015c6269dc
        -368 ┊ <core::iter::adapters::flatten::FlatMap<I,U,F> as core::iter::traits::iterator::Iterator>::next::h0ae42a3ccb2c6b8c
        -368 ┊ <core::iter::adapters::flatten::FlatMap<I,U,F> as core::iter::traits::iterator::Iterator>::next::hd1d6ffb3f2815b1c
        +365 ┊ core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once::he1914ceb08bb5142
        +357 ┊ smoldot::network::protocol::identify::build_identify_response::h0f423b558b108b1e
        -357 ┊ smoldot::network::protocol::identify::build_identify_response::hd1946f888a0df24d
        -324 ┊ alloc::sync::Arc<[T]>::from_iter_exact::hf518ba8d054e3386
        -323 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h9e599229239ebea2
        +297 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h68baf063185e3f2f
        -288 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h706062332f522008
        -288 ┊ <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::h7088c44cb47bbc97
        -285 ┊ alloc::vec::Vec<T,A>::extend_desugared::h1aaa66a0dbd41ed9
        -815 ┊ ... and 102 more.
       -5189 ┊ Σ [122 Total Rows]