Remove psa_crypto_storage_file

Now that we have ITS over files, we no longer need a direct backend
for key storage over files. Remove psa_crypto_storage_file and its
tests.

Switch MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C and MBEDTLS_PSA_ITS_FILE_C on
by default. This preserves functionality and test coverage in the
default configuration, but forgets any key previously stored using the
file backend.

configs/config-psa-crypto.h

@@ -2719,26 +2719,11 @@
  *
  * Module:  library/psa_crypto_storage.c
  *
- * Requires: MBEDTLS_PSA_CRYPTO_C and one of either
- * MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C or MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
- * (but not both)
+ * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
  *
  */
 #define MBEDTLS_PSA_CRYPTO_STORAGE_C
 
-/**
- * \def MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
- *
- * Enable persistent key storage over files for the
- * Platform Security Architecture cryptography API.
- *
- * Module:  library/psa_crypto_storage_file.c
- *
- * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_FS_IO
- *
- */
-#define MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
-
 /**
  * \def MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
  *
@@ -2747,10 +2732,23 @@
  *
  * Module:  library/psa_crypto_storage_its.c
  *
- * Requires: MBEDTLS_PSA_CRYPTO_C, MBEDTLS_PSA_HAS_ITS_IO
+ * Requires: MBEDTLS_PSA_CRYPTO_C,
+ *           either MBEDTLS_PSA_HAS_ITS_IO or MBEDTLS_PSA_ITS_FILE_C
  *
  */
-//#define MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
+#define MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
+
+/**
+ * \def MBEDTLS_PSA_ITS_FILE_C
+ *
+ * Enable the emulation of the Platform Security Architecture
+ * Internal Trusted Storage (PSA ITS) over files.
+ *
+ * Module:  library/psa_its_file.c
+ *
+ * Requires: MBEDTLS_FS_IO
+ */
+#define MBEDTLS_PSA_ITS_FILE_C
 
 /**
  * \def MBEDTLS_RIPEMD160_C