chore: Fix unused warnings on default features

[aborgna-q]

Fixes two unused import errors that only appeared when ignoring some features.
We test cargo clippy --all-features on CI, so this didn't get flagged.

warning: unused import: `poly_func::PolyFuncTypeBase`
  --> hugr-core/src/types.rs:19:16
   |
19 | pub(crate) use poly_func::PolyFuncTypeBase;
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: unused import: `signature::FuncTypeBase`
  --> hugr-core/src/types.rs:21:16
   |
21 | pub(crate) use signature::FuncTypeBase;
   |                ^^^

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.62%. Comparing base (87150fa) to head (ff60b97).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1618   +/-   ##
=======================================
  Coverage   85.62%   85.62%           
=======================================
  Files         136      136           
  Lines       24928    24929    +1     
  Branches    21854    21855    +1     
=======================================
+ Hits        21344    21346    +2     
  Misses       2476     2476           
+ Partials     1108     1107    -1     

Flag	Coverage Δ
python	92.45% <ø> (ø)
rust	84.66% <ø> (+<0.01%)	⬆️

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