[NET]: Remove NETIF_F_INTERNAL_STATS, default to internal stats.

Herbert Xu conviced me that a new flag was overkill; every driver
currently overrides get_stats, so we might as well make the internal
one the default.  If someone did fail to set get_stats, they would now
get all 0 stats instead of "No statistics available".

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>