Revert "flow: Split flow_extract"

It was planned to use this code to allow further processing of packets, a
second pass done when constructing a flow.  Instead it is now planned to
use recirculation to address the problems that secondary processing aimed
to resolve.  As a result there are no longer plans to use
flow_extract_l3_onwards() and it seems prudent to remove it.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>