ACPI: Clean up some S3 related leftovers

With RELOCATABLE_RAMSTAGE the backing up of low memory
on S3 resume path was dropped. We forgot some things
behind.

Change-Id: I674f23dade0095e64619af0ae81e23368b1ee471
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42422
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/program_loading.h

@@ -174,10 +174,6 @@ int legacy_romstage_selector(struct prog *romstage);
 /* Run ramstage from romstage. */
 void run_ramstage(void);
 
-/* Backup OS memory to CBMEM_ID_RESUME on ACPI S3 resume path,
- * if ramstage overwrites low memory. */
-void backup_ramstage_section(uintptr_t base, size_t size);
-
 /***********************
  *   PAYLOAD LOADING   *
  ***********************/