Thin out Flourish code

What does this MR do?

Makes Flourish code less overwhelming by removing lots of unused code.

To be extra sure I don't break anything indirectly (by removing code that classes we use rely on within Flourish), I applied:

• Project wide text search (for class names, function names)
• Static analysis to minimize code based on some fair assumptions
  • We are never going to downgrade to PHP 5
  • This flourish code will never run on anything that is not Linux for us
  • No new merged code will rely on additional Flourish functionality

How confident are you it won't break things if deployed?

Very confident, as long as tests pass. See #999 for classes still used by our code

Links to related issues

#999

How to test

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes refactoring, no user facing changes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.