[TASK] Cleanup EXT/requiredExt handling

LocalConfiguration setting EXT/requiredExt was obsoleted with the
new package manger, the setting is removed by the PackageStates
migration code in install tool already.

* Remove further mentions of this setting
* Adapt method in ExtensionManagementUtility
* Streamline and remove some related units tests
* Move deletion of requiredExt setting to silent update
* Adapt an em view helper to new API

Change-Id: Ie9593c9791893bb4fb0714032efbfc06808b0a35
Resolves: #55555
Releases: 6.2
Reviewed-on: https://review.typo3.org/27232
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Daniel Siepmann
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn