[TASK] Remove TSFE->getDocumentTemplate

Calling the protected method getDocumentTemplate() within TSFE
was added while cleaning up to be PHP-Storm-green in 2015, without
checking if this function is actually in use.

Resolves: #81199
Releases: master
Change-Id: I9a118c9cd20c9c00e836473d96132d8cb8898c7a
Reviewed-on: https://review.typo3.org/52783


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>