Remove psa_crypto_storage_backend.h

Since there is now a single storage backend, we don't need a backend
interface. Make the functions that were declared in
psa_crypto_storage_backend.h and are now both defined and used in
psa_crypto_storage.c static, except for psa_is_key_present_in_storage
which is used by the gray-box tests and is now declared in
psa_crypto_storage.h.

library/psa_crypto_storage.h

diff --git a/library/psa_crypto_storage.h b/library/psa_crypto_storage.h
index 7e5aae9..902e302 100644
--- a/library/psa_crypto_storage.h
+++ b/library/psa_crypto_storage.h
@@ -62,6 +62,21 @@
 #define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xfffeffff
 
 /**
+ * \brief Checks if persistent data is stored for the given key slot number
+ *
+ * This function checks if any key data or metadata exists for the key slot in
+ * the persistent storage.
+ *
+ * \param key           Persistent identifier to check.
+ *
+ * \retval 0
+ *         No persistent data present for slot number
+ * \retval 1
+ *         Persistent data present for slot number
+ */
+int psa_is_key_present_in_storage( const psa_key_file_id_t key );
+
+/**
  * \brief Format key data and metadata and save to a location for given key
  *        slot.
  *