[TASK] Cleanup namespace definitions in EXT:impexp templates

This removes an invalid attribute from EXT:impexp
default layout and furthermore removes unnecessary
fluid namespace declarations, since they are defined
globally.

Resolves: #94306
Releases: master
Change-Id: I0ec7181ccc5fb08708257499c969c1d27e783338
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69456


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>