datapath: Remove implementation of port groups.

datapath: Remove implementation of port groups.

The "port group" concept seems like a good one, but it has not been
used very much in userspace so far, so before we commit ourselves to
a frozen API that we must maintain forever, remove it.  We can always
add it back in later as a new kind of vport.

Signed-off-by: Ben Pfaff <blp@nicira.com>