gtk: drop gtk_widget_set_double_buffered call

Dunno why it is here.  Removing it seems to have no ill side effects.
It is depricated in 3.14+.  In some cases it has no effect since 3.10
according to the docs:

https://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-set-double-buffered



Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>