mhnsw: remove a redundant loop and ha_update_row

sql/vector_mhnsw.cc

@@ -399,25 +399,18 @@ static int update_second_degree_neighbors(MHNSW_Context *ctx, size_t layer,
                                           uint max_neighbors,
                                           const FVectorNode &node)
 {
-  for (const FVectorNode &neigh: node.get_neighbors(layer)) // XXX why this loop?
-  {
-    neigh.get_neighbors(layer).push_back(&node, &ctx->root);
-    if (int err= write_neighbors(ctx, layer, neigh))
-      return err;
-  }
-
   for (const FVectorNode &neigh: node.get_neighbors(layer))
   {
+    neigh.get_neighbors(layer).push_back(&node, &ctx->root);
     if (neigh.get_neighbors(layer).elements > max_neighbors)
     {
-      // shrink the neighbors
       if (int err= select_neighbors(ctx, layer, neigh,
                                     neigh.get_neighbors(layer), max_neighbors))
         return err;
+    }
     if (int err= write_neighbors(ctx, layer, neigh))
       return err;
   }
-  }
 
   return 0;
 }