net: shell: Add cmd to remove IPv6 neighbor from cache

Add "net nbr rm <ipv6 address>" command. This is useful when
debugging IPv6 neighbor discovery procedure.

Change-Id: Idf1d5628bcf01489ca28a77e9a2f84a8ed041c9a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>