arch/x86: Drop CBMEM_TOP_BACKUP

Code has evolved such that there seems to be little
use for global definition of cbmem_top_chipset().
Even for AMD we had three different implementations.

Change-Id: I44805aa49eab526b940e57bd51cd1d9ae0377b4b
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43326
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/Makefile.inc

@@ -164,7 +164,6 @@ romstage-y += post.c
 # gdt_init.S is included by entry32.inc when romstage is the first C
 # environment.
 romstage-y += gdt_init.S
-romstage-y += cbmem.c
 romstage-y += cpu_common.c
 romstage-$(CONFIG_IDT_IN_EVERY_STAGE) += exception.c
 romstage-$(CONFIG_IDT_IN_EVERY_STAGE) += idt.S