feat: disable modexp

[enitrat]

Time spent on this PR:

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Resolves #1422

What is the new behavior?

• Disables the external call to the cairo1 class for modexp operations
• Removes the legacy modexp files.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.9%. Comparing base (221b976) to head (a8ca5ff).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/kakarot/precompiles/precompiles.cairo	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1429     +/-   ##
=======================================
+ Coverage   62.3%   62.9%   +0.6%     
=======================================
  Files         51      49      -2     
  Lines       8591    8332    -259     
=======================================
- Hits        5355    5247    -108     
+ Misses      3236    3085    -151     

Flag	Coverage Δ
	62.9% <0.0%> (?)

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[obatirou]

Update the documentation
Else seem gtm