swarm-controller: refuse a link over any stored object, decodable or not

The existence check read the path as the route's credential type, so an
object stored there that no longer decodes as one answered 500 instead of
409. It now reads the path untyped: anything stored holds the name.

swarm-controller/src/matrix_account.rs

@@ -200,9 +200,7 @@ async fn link_matrix(
         tracing::warn!(path, error = ?e, "linking the matrix account failed");
         Box::new(e.problem(existing))
     };
-    refuse_linked::<matrix::Credential>(store, path)
-        .await
-        .map_err(refused)?;
+    refuse_linked(store, path).await.map_err(refused)?;
     let (token, homeserver, user_id) = resolve_credential(req).await?;
     link(
         store,