[BUGFIX] FlashMessageService not available in TYPO3 6.0

Issue #51079 introduced the disposal of FlashMessageService
which has been added for TYPO3 6.1 the first time. It's obvious
that the current implementation fails in TYPO3 6.0.

Resolves: #53079
Releases: 6.0
Change-Id: I78ee1f844db9069bfbfa8d1e5853f5c005133969
Reviewed-on: https://review.typo3.org/30039
Reviewed-by: Markus Klein
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader