[TASK] Deprecate TemplateService->fileContent

The fileContent method is not used anymore, and can cleanly
be deprecated. It should not be part of the TemplateService
as it is not related to that very much anymore.

Extension authors should implement this method on their own.

Resolves: #77477
Releases: master
Change-Id: I00cba71d602bfda2a48cf6aa9cee00bb857c7168
Reviewed-on: https://review.typo3.org/49380


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>