[BUGFIX] Remove ExtJS normalization

To avoid conflicts with normalize.css, we remove the normalization done by
ExtJS, which is loaded after the normalize.css now.

Resolves: #56418
Releases: 6.2
Change-Id: I8a5b633244c5074755d478d0497b8f438ef86b65
Reviewed-on: https://review.typo3.org/28069
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Frans Saris
Tested-by: Frans Saris