[TASK] Docheader drop-shadow

Crisps up sections by removing Docheader drop shadow.
Improves hierarchy: Strips docheader from module menu.

Change-Id: I96ceaa4c2080740a77704c57a453165debf1d036
Resolves: #49597
Releases: 6.2
Reviewed-on: https://review.typo3.org/21808
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters