Revert "[BUGFIX] Allow to send multiple HTTP headers with the same name"

This reverts commit 1683f107.

Reason for revert:
This breaks existing code, as multiple headers are now
sent to the client which were previously overridden.

As long as there is existing code in TYPO3 where header() is
sent, this change can only be done as a breaking change.

Change-Id: Id7444b1455855702143b3fde5629203f252ba07e
Reverts: #88420
Resolves: #88457
Releases: master, 9.5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60839


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Markus Poerschke <markus@poerschke.nrw>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Benni Mack <benni@typo3.org>