[BUGFIX] isHiddenPalette removes 'form-section' class

If the first palette of a form is hidden, a tiny line is append
under the tab title. This patch removes it.

Resolves: #78327
Releases: master, 7.6
Change-Id: Ibf0f3b9ed243a3d30de9bb726b17ffdec8414c65
Reviewed-on: https://review.typo3.org/50295


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jérémie Roulin <jeremie@hemmer.ch>
Tested-by: Jérémie Roulin <jeremie@hemmer.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>