[TASK] Remove rewrittenPropertyMapper setting in sysext

After enabling the rewrittenPropertyMapper per default, the
explicit enabling in sysexts is no longer needed.

Resolves: #46294
Releases: 6.1
Change-Id: I7f5760ae85999495606a3d56098cf7b38682b452
Reviewed-on: https://review.typo3.org/18919
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn