[BUGFIX] fix javascript error "TBE_EDITOR not defined" in sys_action

This fixes the error "TBE_EDITOR not defined" which occured when using
sys_action > create backend user and assigning a db mount.

Resolves: #34544
Releases: 6.2, 6.1, 6.0
Change-Id: Idb78c23e2dea576fb1cab41979d4b068ca45a8b7
Reviewed-on: https://review.typo3.org/25500
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters