Remove any reference to non-free fonts.

	* commands/videotest.c (grub_cmd_videotest): Use unifont by default.
	* docs/gfxmenu-theme-example.txt: Removed. It's both outdated and
	uses non-free components.
	* font/font.c (grub_font_get_name): Remove example name.
	* gfxmenu/gui_label.c (grub_gui_label_new): Use unifont by default.
	* gfxmenu/gui_list.c (grub_gui_list_new): Likewise.
	* gfxmenu/gui_progress_bar.c (grub_gui_progress_bar_new): Likewise.
	* gfxmenu/view.c (grub_gfxmenu_view_new): Likewise.