Remove dead code for custom nilable types

[yuxincs]

We do not really have custom nilable types for a really long time now, so it makes sense to remove the dead code.

If we ever want to add such custom nilable types again, we should consider adding a hook here via the hook framework (#278).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.69%. Comparing base (ca8aade) to head (3fcfb27).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   87.66%   87.69%   +0.03%     
==========================================
  Files          66       66              
  Lines        7943     7939       -4     
==========================================
- Hits         6963     6962       -1     
+ Misses        798      796       -2     
+ Partials      182      181       -1     

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[github-actions]

Golden Test

Note

✅ NilAway errors reported on standard libraries are identical.

3263 errors on base branch (main, ca8aade)
3263 errors on test branch (e4f8d66)

[sonalmahajan15]

Agree with the code cleanup