[BUGFIX] RTE: JS errors on unload

Cleanup the cleanup operations.

Releases: master
Resolves: #63971
Change-Id: I81706a18cdbd39f13d78fdf9738720682ab56d71
Reviewed-on: http://review.typo3.org/35557
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>