[aat] Rely on vector auto initialization

src/hb-aat-map.hh

@@ -56,15 +56,7 @@ struct hb_aat_map_builder_t
 
   HB_INTERNAL hb_aat_map_builder_t (hb_face_t *face_,
 				    const hb_segment_properties_t *props_ HB_UNUSED) :
-				      face (face_)
-  {
-    features.init ();
-  }
-
-  ~hb_aat_map_builder_t (void)
-  {
-    features.fini ();
-  }
+				      face (face_) {}
 
   HB_INTERNAL void add_feature (hb_tag_t tag, unsigned int value=1);