[TASK] Avoid use of Ext.isDefined

Avoid useless overhead.

Releases: master
Resolves: #62609
Change-Id: I6b2f2805945d0e6a04b3617807f952cd30d8ede8
Reviewed-on: http://review.typo3.org/33700
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>