[IPV6]: Remove three unused method declarations in include/net/ipv6.h

This patch removes three unused method declarations in include/net/ipv6.h:
inet_getfrag_t(), ipv6_build_nfrag_opts() and ipv6_build_frag_opts().

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>