ip6_input: remove redundant v4mapped check

In ip6_input() we apply the same v4mapped address check twice. The only
case which skipps the first one is M_FASTFWD_OURS which should have passed
the check on the firstinput pass and passed the firewall.
Remove the 2nd redundant check.

Reviewed by: kp, melifaro
MFC after: 2 weeks
Sponsored by: Netflix (originally)
Differential Revision: https://reviews.freebsd.org/D22462