[TASK] Remove leftover tt_content.select_key in content preview

The StandardContentPreviewRenderer still contained
special handling for the tt_content.select_key field,
which however was already removed in #77934.

The leftover is now removed together with related
locallang labels.

Resolves: #98125
Related: #77934
Releases: main, 11.5
Change-Id: I38ae07a4eced0cbd3f5ed3076661b70081252321
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75420


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>