Remove unused experimental ICMP6 redirect low water bits

Remove unused experimental ICMP6 redirect low water bits

Dead since introduction in 2001 with icmp6.c r1.31:
    implement upper limit to icmp6 redirects (experimental, turned off)
    negative value to {mtudisc,redirect}_{hi,lo}wat will turn off the limitation.
    sync with kame.

icmp6_redirect_lowat was always -1 and never hit the empty conditional.

icmp6_redirect_hiwat never existed.

icmp6_mtudisc_{hi,lo}wat are exposed as net.inet6.icmp6.mtudisc_{hi,lo}wat
sysctl(2)s, so don't touch those for now.

OK mvs