network: remove unnecessary fd saved in fd-store

network: remove unnecessary fd saved in fd-store

Fixes the following warning:
```
[  130.968170] H systemd-networkd[4288]: Restart operation initiated.
[  130.968265] H systemd-networkd[4288]: Serializing...
[  130.968274] H systemd-networkd[4288]: There is nothing to serialize.
(snip)
[  130.978756] H systemd[1]: Stopped systemd-networkd.service.
(snip)
[  130.991407] H systemd[1]: Starting systemd-networkd.service...
(snip)
[  131.077440] H systemd-networkd[4318]: Enumeration completed.
[  131.077471] H systemd-networkd[4318]: Deserializing...
[  131.077491] H systemd-networkd[4318]: Failed to parse json (line=0, column=0): No data available
[  131.077524] H systemd-networkd[4318]: Failed to deserialize the previous invocation, ignoring: No data available
```