Revert "[BUGFIX] Load XML files of Extension Manager properly"

This reverts commit 3d449f91.

This change did not completely solve the libxml issue and introduced
additional trouble on memory consumption with the current pull parsers.

Related: #75022
Releases: master, 7.6, 6.2
Change-Id: I42454d61ab444860fc53b29715df69e65a2f0475
Reviewed-on: https://review.typo3.org/48182


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>