net: ipv6: Add util to remove neighbor from cache

Add net_ipv6_nbr_rm() utility function that can be used to
remove an IPv6 neighbor from the cache.

Change-Id: I9794856a4f65c5e943656970648e5c5762b0338c
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>