[TASK] RTE: Cleanup references to AllowClipboardHelper for Mozilla

Support for allowClipboard policy has been removed from Firefox
since version 29.
This is a follow-up to change https://review.typo3.org/#/c/33367/

Releases: master, 6.2
Resolves: #62296
Change-Id: I71941c911bc9596495dfea5839d3dba678463357
Reviewed-on: http://review.typo3.org/33754
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>