chore: remove some allow(dead_code) annotations

[bnjbvr]

We have quite a few allow(dead_code) annotations. While it's OK to use in situations where the Cargo-feature combination explodes and makes it hard to reason about when something is actually used or not, in other situations it can be avoided, and show actual, dead code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.11%. Comparing base (618e472) to head (da8b46a).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4472      +/-   ##
==========================================
+ Coverage   85.07%   85.11%   +0.04%     
==========================================
  Files         283      283              
  Lines       31775    31763      -12     
==========================================
+ Hits        27033    27036       +3     
+ Misses       4742     4727      -15     

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[Hywan]

Thanks for the clean up.