net/iface: Remove useless attributes in net_if_ipv6 structure
net/iface: Remove useless attributes in net_if_ipv6 structure

DAD use dad_count attribute in struct net_if_addr, since DAD is ran on
each and every ipv6 unicast address.

Fixes #8728

Signed-off-by: Tomasz Bursztyka <ba81a3a719836727e6857ae83462b9f52ec41006@linux.intel.com>

Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr