Remove serde deny_unknown_fields from client

There was one use of the feature, but it is not needed since it is a
struct for the input of an RPC and not the return.

Remove the feature gate.

Author: jamillambert

client/src/client_sync/v21/mod.rs

@@ -192,7 +192,6 @@ crate::impl_client_v17__get_zmq_notifications!();
 
 /// Request object for the `importdescriptors` method.
 #[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
-#[serde(deny_unknown_fields)]
 pub struct ImportDescriptorsRequest {
     /// Descriptor to import.
     #[serde(rename = "desc")]