[TASK] Cleanup FactoryConfiguration for saltedpasswords

This patch rewrites the configuration of ext:saltedpasswords into
a readable array which gets serialized afterwards. The check for
enabling salted passwords in the backend is obsolete and removed.
The values of the "checkConfiguration" user functions superfluous
and removed as well.

Resolves: #51354
Releases: 6.2
Change-Id: I539070d1642660ad0a5641d8971bdabc7cf92223
Reviewed-on: https://review.typo3.org/23366
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters