device: don't require a hardware address for DHCPv6

DHCPv4 requires a hardware address, while DHCPv6 does not.

Anyway, the DHCP manager already checks that an address is available
when needed, so drop the check here.

Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')

54 jobs for bg/dhcp6-hwaddr in 35 minutes and 56 seconds (queued for 5 seconds)