:rewind: [#4375] Remove disable hidden fields envvar for objects API

[stevenbal]

Closes #4375

Changes

• Remove disable hidden fields envvar for objects API

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [x] Commit messages refer to the relevant Github issue
  • [x] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.56%. Comparing base (bfc679d) to head (8b3c1e3). Report is 18 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4759 +/- ## ========================================== - Coverage 96.56% 96.56% -0.01% ========================================== Files 746 746 Lines 25202 25193 -9 Branches 3318 3316 -2 ========================================== - Hits 24336 24327 -9 Misses 602 602 Partials 264 264 ```

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