[BUGFIX] RTE: Remove cut, copy and paste buttons in Firefox

Support for allowClipboard policy has been dropped by Firefox 29.
Therefore, remove cut, copy and paste buttons from the RTE toolbar
in Firefox. Remove also dialogue for installing AllowClipboard Helper
Firefox Addon.

Releases: master, 6.2
Resolves: #62296
Change-Id: I772b07cdca148c60ef8f61097f96fe5872fc0309
Reviewed-on: http://review.typo3.org/33367
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>