Remove device: eth0 from dummy interface

With eth0 specified as the default interface device, all interfaces by default
were attempted to be renamed to eth0. This didn't break before because device
renaming wasn't working for multiple interfaces in general; but now that it's
fixed, we should no longer be setting a default interface name.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

docs/man/man5/defaults.conf.5

@@ -30,7 +30,6 @@ defaultnode:
   - default
   network devices:
     dummy:
-      device: eth0
       type: ethernet
       netmask: 255.255.255.0
 .EE