[TASK] Remove broken include_static.txt TypoScript include

The inclusion of 'include_static.txt' files within the
TypoScript inclusion magic has been part of the extension
'statictemplates' back in the old days: It brought a database
field 'include_static' together with a hook in TemplateService
that took care of that field content. It then indirectly also
handled these 'include_static.txt' file inclusions.

Extension 'statictemplates' has been removed from core with
TYPO3 6.2. Since then, 'include_static.txt' files had no
effect anymore.

The patch removes the dead code artifact from TemplateService
and adapts some documentation accordingly.

Resolves: #97501
Related: #46469
Releases: main
Change-Id: I07bb118bc405c97da6810eca0061ab6968564fc7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74434


Reviewed-by: Stephan Großberndt <stephan.grossberndt@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benjamin Franzke <bfr@qbus.de>