[CLEANUP] Improve checkboxes in EXT:scheduler

The checkboxes in EXT:scheduler are too big. Change the CSS class to
fix this.

Resolves: #63126
Releases: master
Change-Id: I2fa43cb14fb4c67967208c59703b584dd832b3b8
Reviewed-on: http://review.typo3.org/34440
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>