Remove dead ST31 code and refactor ST31/ST33 defines

[yrichard-ledger]

Description

Cleanup ST arch #ifdefs and remove dead ST31-specific code

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Tests
• Documentation
• Other (for changes that might not fit in any category)

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it.

Additional comments

Please post additional comments in this section if you have them, otherwise delete it.

Auto cherry-pick in API_LEVEL

If requested to port the commits from this PR on a dedicated API_LEVEL branch,
select the targeted one(s), or add new references if not listed:

[ ] TARGET_API_LEVEL: API_LEVEL_26

This will only create the PR with cherry-picks, ready to be reviewed and merged.

Remember:

• The merge will ALWAYS be a manual operation.
• It is possible the cherry-picks don't apply correctly, mainly if previous commits have been forgotten.
• In case of failure, there is no other solution than redo the operation manually...

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (06b034a) to head (7d20289).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1618   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files          39       39           
  Lines        4653     4653           
  Branches      596      596           
=======================================
  Hits         4275     4275           
  Misses        266      266           
  Partials      112      112           

Flag	Coverage Δ
unittests	91.87% <ø> (ø)

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