AMD GX1: Remove useless copied header file `northbridge.h`

This was there since the beginning

    commit d24d6993b6
    Author: arch import user (historical) <svn@openbios.org>
    Date:   Wed Jul 6 17:06:46 2005 +0000

        Revision: linuxbios@linuxbios.org--devel/freebios--devel--2.0--patch-26
        Creator:  Hamish Guthrie <hamish@prodigi.ch>

        Added AMD GX1 northbridge and cs5530 Southbridge

but blindly copied from Intel 440 BX and is not used anywhere.

Thanks to Idwer Vollering for spotting this.

Change-Id: I38b3d3feb25966c3aa382994d323e59c3f3c9e6c
Reported-by: Idwer Vollering
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3020
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)

src/northbridge/amd/gx1/northbridge.c

@@ -6,7 +6,6 @@
 #include <device/pci_ids.h>
 #include <stdlib.h>
 #include <string.h>
-#include "northbridge.h"
 #include <cpu/amd/gx1def.h>
 #include <cpu/x86/cache.h>
 #include <cpu/cpu.h>

src/northbridge/amd/gx1/northbridge.h

@@ -1,6 +0,0 @@
-#ifndef NORTHBRIDGE_INTEL_440BX_H
-#define NORTHBRIDGE_INTEL_440BX_H
-
-extern unsigned int i440bx_scan_root_bus(device_t root, unsigned int max);
-
-#endif /* NORTHBRIDGE_INTEL_440BX_H */