Kimchi/expr: misc cleaning + remove unused methods

[dannywillems]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 13.33333% with 13 lines in your changes missing coverage. Please review.

Project coverage is 72.53%. Comparing base (ff3359e) to head (3736db0).
Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
kimchi/src/circuits/expr.rs	13.33%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2707      +/-   ##
==========================================
+ Coverage   72.51%   72.53%   +0.01%     
==========================================
  Files         246      246              
  Lines       58268    58233      -35     
==========================================
- Hits        42255    42239      -16     
+ Misses      16013    15994      -19     

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