net: ipv6: Remove unused function

Building with clang warns:

subsys/net/ip/ipv6_nbr.c:137:31: error: unused function
'get_nbr_from_data' [-Werror,-Wunused-function]
static inline struct net_nbr *get_nbr_from_data(struct net_ipv6_nbr_data
                              ^                 *data)

Signed-off-by: Tom Hughes <tomhughes@chromium.org>