net: kill inpgroups

"inpgroups" are some bsd mechanism used to group together connections
sharing some affinity to a thread/cpu.  We don't enable it however, so
it's dead code and hampers further development of the network stack.

Remove it.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tcp: Remove the function tcp_twrecycleable

This is a backport from freebsd:

   commit 278abd32202c5e621d3d868824000d2f296645f4
   Author: silby <silby@FreeBSD.org>
   Date:   Fri May 16 01:38:38 2014 +0000

       Remove the function tcp_twrecycleable; it has been #if 0'd for
       eight years.  The original concept was to improve the
       corner case where you run out of ephemeral ports, but it
       was causing performance problems and the mechanism
       of limiting the number of time_wait sockets serves
       the same purpose in the end.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>