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.
diff --git a/scripts/config.pl b/scripts/config.pl
index 624deca..5542b2d 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -103,7 +103,6 @@
 MBEDTLS_PSA_CRYPTO_SPM
 MBEDTLS_PSA_HAS_ITS_IO
 MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER
-MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
 MBEDTLS_USE_PSA_CRYPTO
 _ALT\s*$
 );
@@ -125,9 +124,9 @@
 MBEDTLS_PLATFORM_TIME_ALT
 MBEDTLS_PLATFORM_FPRINTF_ALT
 MBEDTLS_PSA_CRYPTO_STORAGE_C
-MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C
 MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C
 MBEDTLS_PSA_HAS_ITS_IO
+MBEDTLS_PSA_ITS_FILE_C
 );
 
 # Things that should be enabled in "full" even if they match @excluded