[BUGFIX] EM: Fetch list as html, not as json

After performing an update from TER currently the extension-list
is fetched as format=json. This however leads to that situation
that links in the list also have format=json and won't work
anymore (example: pagination-widget).

Change-Id: Id37ab27bb3acd821e9dc0cadb23978aa3e5d9784
Resolves: #53423
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/25269
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind