flow: Move flow_extract_stats() to dpif.c, as dpif_flow_stats_extract().

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.