flow: Move flow_extract_stats() to dpif.c, as dpif_flow_stats_extract().
The "flow" module is concerned only with OpenFlow flows these days. It
shouldn't have anything to do with ODP or dpifs. However, it included
dpif.h just to implement flow_extract_stats(). This function is a better
fit for dpif.c, so this commit moves it there and removes the dpif.h
#include from flow.h and flow.c
This commit also removes a few more dpif.h #includes that weren't needed.