[BUGFIX] RTE: Inconsistent delete behaviour

In Firefox, depending on how we delete something in RTE, we get
different results.
This change makes the results consistent and identical in all browsers.

Releases: master, 6.2
Resolves: #61747
Change-Id: I7bc15992efb0703b96a59adf6d14484249d9f785
Reviewed-on: http://review.typo3.org/33498
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>