[TASK] ext:saltedpasswords: Remove isUsageEnabled for backend

Remove all calls on SaltedPasswordsUtility::isUsageEnabled('BE')
as backend is enabled by default. Besides remove all
ExtensionManagementUtility::isLoaded('saltedpasswords') as
extension is loaded by default.

Change-Id: Ie2332fc3c6c454888afc8c9956b9869309623584
Resolves: #51356
Releases: 6.2
Reviewed-on: https://review.typo3.org/23375
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters