treeview: fix dragging of columns in mutter

Don't recurse the mainloop in _gtk_tree_view_column_start_drag().
It doesn't serve any discernible purpose, and recursing the
mainloop from the flush-events phas of the frame clock breaks
frame synchronization with mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=705176

gtk/gtktreeview.c

@@ -9926,8 +9926,6 @@ _gtk_tree_view_column_start_drag (GtkTreeView       *tree_view,
   gdk_window_show (tree_view->priv->drag_window);
 
   gtk_widget_grab_focus (GTK_WIDGET (tree_view));
-  while (gtk_events_pending ())
-    gtk_main_iteration ();
 
   tree_view->priv->in_column_drag = TRUE;