[BUGFIX] Cleanup in SelectTreeElement

* Remove unnecessary $treeData variable
* Fix broken JS in onChange handler
* Enforce int casting of $width

Resolves: #70272
Releases: master
Change-Id: I76b2233a30f6c033839f53404340485f7302f9ca
Reviewed-on: http://review.typo3.org/43697


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Andreas Bouche <andreas.bouche@flagbit.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>