soc/intel/common: 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: I4cdf64119e9bfa377520d7e343434f5a6ddab3a8
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50535
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/soc/intel/common/block/acpi/acpi.c

@@ -19,7 +19,6 @@
 #include <soc/gpio.h>
 #include <soc/iomap.h>
 #include <soc/pm.h>
-#include <string.h>
 
 #define  CPUID_6_EAX_ISST	(1 << 7)