modules: TF-M: Remove tfm_strlen.h

The header does not exist anymore with TF-M v2.3.

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>

Author: Vge0rge

modules/trusted-firmware-m/tfm_boards/common/attest_hal.c

@@ -13,7 +13,6 @@
 #include "tfm_attest_hal.h"
 #include "tfm_plat_boot_seed.h"
 #include "tfm_plat_device_id.h"
-#include "tfm_strnlen.h"
 #include "nrf_provisioning.h"
 #include <bl_storage.h>