[TASK] Remove TypoScript option from fixture extension

The TypoScript option rewrittenPropertyMapper is not of any
use anymore and can safely be removed.

Resolves: #61864
Releases: master
Change-Id: Ib9c76be598b24858e58b7e2a2e7bb7fe3acc3605
Reviewed-on: http://review.typo3.org/32961
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>