nb/intel: Remove unused <string.h>

Found using:
diff <(git grep -l '#include <string.h>' -- src/) <(git grep -l 'STRINGIFY\|memcpy\|memmove\|memset\|memcmp\|memchr\|strdup\|strconcat\|strnlen\|strlen\|strchr\|strncpy\|strcpy\|strcmp\|strncmp\|strspn\|strcspn\|strstr\|strtok_r\|strtok\|atol\|strrchr\|skip_atoi\|snprintf' -- src/)|grep '<'

Change-Id: Ica0354fdfe6861551689e3baef94d364d9ded16d
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50690
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/northbridge/intel/haswell/gma.c

@@ -16,7 +16,6 @@
 #include <cpu/intel/haswell/haswell.h>
 #include <drivers/intel/gma/opregion.h>
 #include <southbridge/intel/lynxpoint/pch.h>
-#include <string.h>
 #include <types.h>
 
 #include "chip.h"

src/northbridge/intel/sandybridge/raminit_common.c

@@ -4,7 +4,6 @@
 #include <commonlib/helpers.h>
 #include <console/console.h>
 #include <cpu/intel/model_206ax/model_206ax.h>
-#include <string.h>
 #include <device/mmio.h>
 #include <device/pci_ops.h>
 #include <northbridge/intel/sandybridge/chip.h>