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/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 28cdb43..7d6c0c9 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -525,23 +525,12 @@
 #error "MBEDTLS_PSA_CRYPTO_SPM defined, but not all prerequisites"
 #endif
 
-#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C) && defined(MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C)
-#error "Only one of MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C or MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C can be defined"
-#endif
-
 #if defined(MBEDTLS_PSA_CRYPTO_STORAGE_C) &&            \
     !( defined(MBEDTLS_PSA_CRYPTO_C) &&                 \
-       ( defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C) ||  \
-         defined(MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C) ) )
+       defined(MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C) )
 #error "MBEDTLS_PSA_CRYPTO_STORAGE_C defined, but not all prerequisites"
 #endif
 
-#if defined(MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C) &&            \
-    !( defined(MBEDTLS_PSA_CRYPTO_STORAGE_C) &&           \
-       defined(MBEDTLS_FS_IO) )
-#error "MBEDTLS_PSA_CRYPTO_STORAGE_FILE_C defined, but not all prerequisites"
-#endif
-
 #if defined(MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C) &&             \
     ! defined(MBEDTLS_PSA_CRYPTO_STORAGE_C)
 #error "MBEDTLS_PSA_CRYPTO_STORAGE_ITS_C defined, but not all prerequisites"