[TASK] Simplify modal callback handling

The fix from #84499 can be simplified a bit: class
"modal-content-loaded" is not needed with the callback
solution.

Resolves: #84559
Related: #84499
Releases: master
Change-Id: I2900ecafc06579076670f8678471530d03d0b63c
Reviewed-on: https://review.typo3.org/56477


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>