[BUGFIX] Fix invalid button group markup in scheduler

An element with the class `btn-group` must not also have the class `btn`.
Such occurences are fixed in this patch.

Resolves: #79395
Related: #78199
Releases: master
Change-Id: I4793afc984936c2fc7226feac24f03b6ec94794f
Reviewed-on: https://review.typo3.org/51365


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>