remove defunct schema keys

Specifically these keys are no longer defined in dn42.schema objects:
- `owners`
- `network-owners`
- `primary-key`

data/schema/ROUTE6-SCHEMA

@@ -12,6 +12,5 @@ key:                remarks     optional  multiple
 key:                source      required  single    lookup=dn42.registry
 key:                pingable    optional  multiple
 key:                max-length  optional  single
-network-owner:      inet6num
 mnt-by:             DN42-MNT
 source:             DN42