Rm all legacy (WIP)

- Removes legacy code, tests and docs
- Updates a few test, e.g. to work without unittest_toolbox, because
  relevant tempdir/file logic can easily be implemented using
  builtins; same for custom log module.

*TODO:*
- Check some of the removed docs, if we need to preserve anything
  (eg. QUICKSTART.md, ATTACKS.md)
- Check if we need https tests
- Maybe create a ticket for test metadata creation scripts
- Review remaining docs for necessary updates (eg. README.md,
  INSTALLATION.rst)
- Write proper commits
- Mention (in PR) preliminary work on python-tuf, spec, website and
  CII badge
- Add appreciation note for all authors of legacy code

Signed-off-by: Lukas Puehringer <[email protected]>

Author: lukpueh

docs/CLI.md

@@ -91,3 +91,5 @@ package manager, among other options::
 
     $ brew install python3
     $ brew install libffi
+
+.. TODO: Revise