[BUGFIX] Remove native header() call in DocumentTemplate

In DocumentTemplate there is always one header call
to define that everything is HTML and UTF-8.

Since we do PSR-7 in Backend Context, this should be handled
by the Response object, which we already do.

This should normally be a bugfix for existing stable versions,
although I'm very unsure on how this could affect any side-effects
where extensions rely on this behavior for years (!) already, so
this is master-only.

OTOH this change should not affect anything in regular BE
modules as they work with PSR-7 since TYPO3 v7 already.

Resolves: #88743
Releases: master
Change-Id: I20fb4844ca661c47eb1d56b6d20a8f2c58b17796
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61285


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>