[TASK] Delete unused iframepanel JavaScript code

Since the new ViewPort setup the iframepanel JavaScript
code is unused. The new ViewPort code marked this code already
as a breaking change.

Resolves: #78536
Releases: master
Change-Id: I651dbcd99af76e20f36e8eb7d0c29cf22ddd304a
Reviewed-on: https://review.typo3.org/50480


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>