network: change log level when sd_rtnl_message_get_family() returns invalid family...

network: change log level when sd_rtnl_message_get_family() returns invalid family (#6923)

From bce67bbee359eec19e6778619b6651100a1c1477, systemd-networkd always shows
```
rtnl: received address with invalid family type 32, ignoring.
```
during boot-up. In the code, there are log_warning() and log_debug() for the
same situation, and the log_debug() is never called. So, let's lower the
log level and remove never called function.