Remove xen VFB.

Due to XEN bug it prevents Linux boot. Remove it at least, until
	workaround is found.

include/grub/xen.h

@@ -82,15 +82,6 @@ void grub_console_init (void);
 void grub_xendisk_fini (void);
 void grub_xendisk_init (void);
 
-void
-grub_xen_keyboard_init (void);
-void
-grub_xen_keyboard_fini (void);
-void
-grub_video_xen_init (void);
-void
-grub_video_xen_fini (void);
-
 #ifdef __x86_64__
 typedef grub_uint64_t grub_xen_mfn_t;
 #else