net: if: Clear neighbor cache when removing IPv6 addr with active DAD

DAD creates an entry in the neighbor cache for the queried (own)
address. In case the address is removed from the interface while DAD is
still incomplete, we need to remove the corresponding cache entry (just
like in case of DAD timeout) to avoid stale entries in the cache.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit a09fd8e9)