[BUGFIX] Delete form fields from DOM in flexform sections

Deleting flexform sections now removes the fields from DOM as
DataHandler only cares for the "action" being sent. This commit also
fixes a strange condition that checks whether the action array has empty
values.

To have at least one similarity with IRRE, deleting a section now also
fades the container.

Resolves: #71607
Releases: master
Change-Id: Ib696385ff4bdd3daed323a94c5dd7fdd3a74f640
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63830


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>