bigint: Remove redundant ">= 3" check for Modulus.

[briansmith]

As we require the modulus to be multiple limbs long, its value cannot be less than 3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.95%. Comparing base (01742ba) to head (1c861df).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2184   +/-   ##
=======================================
  Coverage   96.94%   96.95%           
=======================================
  Files         154      154           
  Lines       20238    20216   -22     
  Branches      464      462    -2     
=======================================
- Hits        19620    19600   -20     
+ Misses        512      511    -1     
+ Partials      106      105    -1     

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