This PR removes the supplementary groups addition to the service files on Linux.
Turns out that the override does not support variables, so it is not possible to supply the groups this way.
The right way to do this is to run something like:
```
sudo systemctl edit duplicati.service
```
Then add the directive:
```
[Service]
SupplementaryGroups=media backup
```
When saving, an override file will be created, likely in:
```
/etc/systemd/system/duplicati.service.d/override.conf
```
You can now reload the daemon, and the override will persist even on package upgrades.
Kenneth Skovhede
805489cf6e