[TASK] Cleanup fluid

- Remove dead code
- Add missing property definitions
- Add function descriptions

Change-Id: I0ec563d42d7e763db7f482c9721edb4717703213
Resolves: #46162
Releases: 6.1
Reviewed-on: https://review.typo3.org/18831
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn