ofproto: Send only one OFPT_FLOW_REMOVED message when a flow expires.

Commit 15aaf59932a3 (ofproto: Add global locking around flow table
changes.) introduced doubled messages for expirations.

Reported-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>