Revert signature-notification format change (#12032)

* Use untagged RpcSignatureResult enum to avoid breaking downstream consumers of current signature subscriptions

* Clean up client duplication

* Clippy

core/tests/rpc.rs

@@ -285,7 +285,7 @@ fn test_rpc_subscriptions() {
         let timeout = deadline.saturating_duration_since(Instant::now());
         match status_receiver.recv_timeout(timeout) {
             Ok((sig, result)) => {
-                if let RpcSignatureResult::ProcessedSignatureResult(result) = result.value {
+                if let RpcSignatureResult::ProcessedSignature(result) = result.value {
                     assert!(result.err.is_none());
                     assert!(signature_set.remove(&sig));
                 } else {