datapath: Drop flow information from odp_stats.

Userspace used to use the n_flows information here to decide how much
memory needed to be allocated to list flows, but that isn't necessary any
longer now that listing flows uses an iterator abstraction.  The
cur_capacity and max_capacity members are just curiosities and don't
provide much information; if the implementation ever changes away from
the current hash table implementation then they could become meaningless
anyhow.

But more than anything, these aren't really the kind of statistics that
networking people usually care about.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>