Clear up some confusion about attribute count and external references

Now the count is always adjacent to whereever the key/value pair is
stored, and is not kept in the serial feature object other than as
the length of the vectors of keys and values.

geojson.cpp

@@ -193,7 +193,6 @@ int serialize_geojson_feature(struct serialization_state *sst, json_object *geom
 	sf.has_tippecanoe_maxzoom = (tippecanoe_maxzoom != -1);
 	sf.tippecanoe_maxzoom = tippecanoe_maxzoom;
 	sf.geometry = dv;
-	sf.m = m;
 	sf.feature_minzoom = 0;  // Will be filled in during index merging
 	sf.seq = *(sst->layer_seq);