Revert "Change PSA_MAX_PERSISTENT_KEY_IDENTIFIER to 64 bits when using ITS"

This reverts commit 7b6f37f.
This revert is due to PR #59.

Author: itayzafrir

library/psa_crypto_storage.h

@@ -59,11 +59,7 @@ extern "C" {
  * This limitation will probably become moot when we implement client
  * separation for key storage.
  */
-#if defined(MBEDTLS_PSA_CRYPTO_SPM)
-#define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xffff000000000000
-#else
 #define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xffff0000
-#endif
 
 /**
  * \brief Format key data and metadata and save to a location for given key