💥 Remove conversion of form step URL to form step UUID

[sergei-maertens]

Part of #3283

Fully qualified URLs to resolve a form step have been deprecated for a while, since they're not portable across environments with different base URLs.

Now the compatibility code is removed too, which may break old exports.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

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

• Release management

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

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

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.60%. Comparing base (ae74347) to head (68a21bb).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4892      +/-   ##
==========================================
- Coverage   96.60%   96.60%   -0.01%     
==========================================
  Files         754      754              
  Lines       25853    25844       -9     
  Branches     3415     3414       -1     
==========================================
- Hits        24976    24967       -9     
  Misses        613      613              
  Partials      264      264              

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