Remove SubField API

[crutchcorn]

This PR removes the subfield API from FieldAPI, as:

1. We're removing the implicit ability to create a new FieldAPI from a parent's name in react-core (Array and provider refactor #588)
2. This API adds a ton of complexity (especially around typings)
3. It's just as easy to new FieldAPI with a new name

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.75%. Comparing base (9602113) to head (8f70742).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #607      +/-   ##
==========================================
- Coverage   87.78%   87.75%   -0.04%     
==========================================
  Files          30       30              
  Lines         794      792       -2     
  Branches      179      179              
==========================================
- Hits          697      695       -2     
  Misses         92       92              
  Partials        5        5              

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[fulopkovacs]

🚢 lgtm!