Merge psa_crypto_storage_its into psa_crypto_storage

Since the ITS API has stabilized and we don't plan to make use of more
than ITS, we don't need an abstraction layer between key storage and
key storage over ITS. Merge the ITS code into the generic storage
module.

include/mbedtls/config.h

@@ -2761,25 +2761,12 @@
  *
  * Module:  library/psa_crypto_storage.c
  *
- * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
- *
+ * Requires: MBEDTLS_PSA_CRYPTO_C,
+ *           either MBEDTLS_PSA_ITS_FILE_C or a native implementation of
+ *           the PSA ITS interface
  */
 #define MBEDTLS_PSA_CRYPTO_STORAGE_C
 
-/**
- * \def MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
- *
- * Enable persistent key storage over PSA ITS for the
- * Platform Security Architecture cryptography API.
- *
- * Module:  library/psa_crypto_storage_its.c
- *
- * Requires: MBEDTLS_PSA_CRYPTO_C,
- *           either MBEDTLS_PSA_HAS_ITS_IO or MBEDTLS_PSA_ITS_FILE_C
- *
- */
-#define MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
-
 /**
  * \def MBEDTLS_PSA_ITS_FILE_C
  *