remove a left over that uselessly fetches all users

Removing this line slipped in https://github.com/owncloud/core/pull/10558 while it was in the original PR https://github.com/owncloud/core/pull/8507. It causes all users to be fetched upon opening admin page when files_external is enabled. The placeholder they are assigned to has been removed in the mentioned PRs.

@butonic @LukasReschke @PVince81