ipv6: remove rt6i_peer_genid from rt6_info and its handler

6431cbc2(Create a mechanism for upward inetpeer propagation into routes)
introduces these codes, but this mechanism is never enabled since
rt6i_peer_genid always is zero whether it is not assigned or assigned by
rt6_peer_genid(). After 5943634f

 (ipv4: Maintain redirect and PMTU info
in struct rtable again), the ipv4 related codes of this mechanism has been
removed, I think we maybe able to remove them now.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>