Remove duplicate unit test in sharing that checks for group permission updates

* it was skipped before anyways
* it is covered for example in build/integration/sharing_features/sharing-v1-part3.feature#L517-L548 (see https://github.com/nextcloud/server/blob/54f8f75f6f82ca102b1cfdb88caf0d06e6c9ba31/build/integration/sharing_features/sharing-v1-part3.feature#L517-L548

)
* more permission updates are tested in the webdav section, where an OCS API is called and the WebDAV response is checked for the properly changed permission

Signed-off-by: Morris Jobke <hey@morrisjobke.de>