net: lwm2m: Cleanup special handling of DNS for offloading

With `CONFIG_NET_NATIVE`, offloaded drivers can specify capabilites with
`NET_IPV4/6` configs, so there is no longer need to handle socket
offloading separately.

Also, initialize hints structure with zeros, as according to man pages
unused fields should be set to 0:
`All the other fields in the structure pointed to by hints must contain
either 0 or a NULL pointer, as appropriate.`

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>