[TASK] Remove deprecated initCharset from DocumentTemplate

Removes also the deprecated method initCharset

Resolves: #56841
Releases: 6.2
Change-Id: I8e94a7ce43075f96bed857bbda4567a5165f5967
Reviewed-on: https://review.typo3.org/28319
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer