[TASK] Remove deprecated prepareVariableForJavascript from DebugUtility

Resolves: #56846
Releases: 6.2
Change-Id: Id769a6fb25cd8c207d2591f6b8f9b95165d04e05
Reviewed-on: https://review.typo3.org/28324
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer