net: remove outdated comment for ipv4 and ipv6 protocol handler

since f9242b6b


inet: Sanitize inet{,6} protocol demux.

there are not pretended hash tables for ipv4 or
ipv6 protocol handler.

Signed-off-by: Baker Zhang <Baker.kernel@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>