Ipset returns code 1 as element cannot be deleted
On our [forum](https://forum.turris.cz/t/turris-os-5-0-0-is-released-in-hbt/12733/109?u=pepe), it was reported to us this following error:
```
sentinel-dynfw-client[7245]: ipset v7.3: Error in line 1: Element cannot be deleted from the set: it's not added
sentinel-dynfw-client[7245]: 2020-05-16 11:45:50,206 - WARNING - Error running ipset command: return code 1.
```
issue