ofproto-dpif: Remove ovs-appctl dpif/del-flows.

ofproto-dpif: Remove ovs-appctl dpif/del-flows.

It's not entirely clear why someone would want to delete the flows
associated with only one bridge and not be fine with simply using
ovs-dpctl del-flows to get all of them.  Additionally, this will be
hard to implement cleanly in the future, so this patch removes it.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>