[!!!][TASK] Remove TCEMAIN.previewDomain

The functionality to set a custom TCEMAIN.previewDomain via
PageTS is not in use anymore as SiteHandling
is using the proper domain anyway already.

Conceptually this does not work anymore, as the base (domain + path prefix)
determines Language + Site / PageTree entry point. Setting
this to something else via TCEMAIN.previewDomain would not work
anyways, so this hack is removed, as we now can safely
determine the full (speaking) URL with Site Handling.

In addition, the method BackendUtility::getViewDomain()
is now deprecated.

Resolves: #88499
Releases: master
Change-Id: Id88c16e2e86ccce8a2e7be02af0e2a39802624c0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60773


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Gorges <daniel.gorges@b13.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Gorges <daniel.gorges@b13.de>
Reviewed-by: Benni Mack <benni@typo3.org>