ofproto-dpif: Remove vestigial "clogged" feature.

ofproto-dpif: Remove vestigial "clogged" feature.

Before this commit, ofproto-dpif did not ordinarily ever defer
completion of a flow_mod, but it was possible to use "ovs-appctl
ofproto/clog" to force deferring completion, which allowed some basic
debugging of deferred completion.  I only ever used that feature once,
while doing initial debugging of the deferred completion feature.  I
intend in future commits to make changes that will make deferred
completion harder to implement in ofproto-dpif, so this commit removes
that feature in advance.

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