Revert "Add custom subtypes function to avoid deprecation warnings"

[KristofferC]

Reverts #236

The added function hangs on 1.12 and the comment to it says

f it starts to cause problems in future version of Julia, it can be removed without any issues

So let's remove it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.43%. Comparing base (56a19bb) to head (120ba24).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #239      +/-   ##
==========================================
- Coverage   69.73%   68.43%   -1.31%     
==========================================
  Files           3        3              
  Lines         456      434      -22     
==========================================
- Hits          318      297      -21     
+ Misses        138      137       -1     

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