Remove needless custom implementation of None.infix==

[kyouko-taiga]

Now that infix== is synthesizable, there is no need to keep None's custom implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.66%. Comparing base (0392eff) to head (9fdd999).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1565      +/-   ##
==========================================
+ Coverage   87.64%   87.66%   +0.01%     
==========================================
  Files         373      373              
  Lines       22614    22620       +6     
==========================================
+ Hits        19821    19829       +8     
+ Misses       2793     2791       -2     

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