[BUGFIX] IRRE forms not using full width

In 4.7 IRRE forms used to be full width of the parent form.
A change introduced in 6.0 now causes the forms to collapse
to less than the full width.

Change-Id: Ie326a339c762c539440e6f70cc51b467e39a666c
Fixes: #44549
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/17662
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner