Fix dead code warning in expanded code

[ggwpez]

Hitting this clippy warning here paritytech/polkadot-sdk#5676 (comment)

error: trait `OrchestraFieldState` is never used
     --> /__w/polkadot-sdk/polkadot-sdk/target/debug/build/expander-f21e58b59c39b7b5/out/orchestra-expansion-d13355006895.rs:14155:7
      |
14155 | trait OrchestraFieldState<T> {}
      |       ^^^^^^^^^^^^^^^^^^^
      |
      = note: `-D dead-code` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(dead_code)]`

    Rustc finished with error: exit status 1
error: could not compile `polkadot-overseer` (lib) due to 1 previous error