Remove unused custom set types

[adrhill]

The custom set types RecursiveSet and SortedVector should live in a separate package.
Deciding to remove them now as they make test fail on pre. Ping @gdalle.

This PR is non-breaking since both types are unexported and undocumented.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.92%. Comparing base (fe3c799) to head (da4d274).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   92.16%   92.92%   +0.75%     
==========================================
  Files          30       28       -2     
  Lines        1200     1088     -112     
==========================================
- Hits         1106     1011      -95     
+ Misses         94       77      -17     

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