remove unused $variables{'os'}

Change-Id: Idaf0b0e9fc38dc05f24c111304ef5189b4ba111c
Reviewed-on: https://gerrit.libreoffice.org/11906
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>