src/{drivers,security}: 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: Ief86a596b036487a17f98469c04faa2f8f929cfc
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50691
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/drivers/intel/usb4/retimer/retimer.c

@@ -6,7 +6,7 @@
 #include <device/device.h>
 #include <device/path.h>
 #include <gpio.h>
-#include <string.h>
+
 #include "chip.h"
 #include "retimer.h"
 

src/security/intel/txt/logging.c

@@ -3,7 +3,6 @@
 #include <arch/mmio.h>
 #include <console/console.h>
 #include <cpu/x86/smm.h>
-#include <string.h>
 #include <types.h>
 
 #include "txt.h"