packet: Kill CONFIG_PACKET_MMAP.

Early on this was an experimental facility that few
people other than Alexey Kuznetsov played with.

Now it's a pretty fundamental thing and as people add
more features to AF_PACKET sockets this config options
creates ifdef spaghetti.

So kill it off.

Signed-off-by: David S. Miller <davem@davemloft.net>