ofproto/trace: Remove the unused variables.

This commit removes the unused variables in ofproto_trace() function.

Signed-off-by: Alex Wang <alexw@nicira.com>
ofproto/trace: Change output field name.

This commit changes the output field name from "Relevant fields"
to "Megaflow".

Signed-off-by: Alex Wang <alexw@nicira.com>
ofproto/trace: Show megaflow field in each resubmit.

This commit makes the ofproto/trace output show the megaflow field
of each resubmit.

Signed-off-by: Alex Wang <alexw@nicira.com>