[TASK] Remove styles for former ExtensionManager

Deleted CSS files that bring styles for former EXT: em.

Change-Id: Ia8bd3c8971ba5aabb7792a08b4cff577d1f95ec6
Resolves: #48550
Releases: 6.2
Reviewed-on: https://review.typo3.org/21073
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters