[BUGFIX] Remove gap underneath Workspace tabs

In Workspace backend module the gap between tabs
and table is removed.

Change-Id: I1031c1a8eed43a23bc87cf7415972b5bd1cd6055
Resolves: #49133
Releases: 6.2
Reviewed-on: https://review.typo3.org/22274
Reviewed-by: Max Roesch
Tested-by: Max Roesch
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters