refactor(rust): Purge unused code

[orlp]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 52.17391% with 11 lines in your changes missing coverage. Please review.

Project coverage is 80.11%. Comparing base (20ead46) to head (bb3ec53).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/pow.rs	31.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19267      +/-   ##
==========================================
+ Coverage   80.03%   80.11%   +0.08%     
==========================================
  Files        1528     1526       -2     
  Lines      209657   209330     -327     
  Branches     2416     2416              
==========================================
- Hits       167791   167707      -84     
+ Misses      41317    41074     -243     
  Partials      549      549              

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[codspeed-hq]

CodSpeed Performance Report

Merging #19267 will improve performances by 25.14%

_{Comparing orlp:unused-purge (bb3ec53) with main (3255066)}

Summary

⚡ 1 improvements
✅ 40 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	orlp:unused-purge	Change
⚡	test_groupby_h2oai_q5	2.7 ms	2.1 ms	+25.14%