ofproto: Get rid of port_changed_cb.

ofproto: Get rid of port_changed_cb.

Jesse pointed out that port_changed_cb isn't a great interface.  It's only
around because, earlier, we had a lousy interface for monitoring netdev
status, so that we needed to pass along information obtained by ofproto
into the bridge.  But netdev_monitor is now sufficiently sophisticated that
the bridge can set up an independent netdev_monitor without any important
loss of efficiency.  Since this makes the code cleaner, this commit does
so.