Use secret_service_clear_sync() in wxSecretStore

There doesn't seem to be any reason to use secret_service_xxx()
functions for Save() and Load(), but use simple API function
secret_password_clearv_sync() for Delete().

Use the equivalent function from the same API layer for it too instead.

src/unix/secretstore.cpp

@@ -209,8 +209,9 @@ public:
                         wxString& errmsg) wxOVERRIDE
     {
         wxGtkError error;
-        if ( !secret_password_clearv_sync
+        if ( !secret_service_clear_sync
               (
+                NULL,                           // Default service
                 GetSchema(),
                 BuildAttributes(service),
                 NULL,                           // Can't be cancelled