Drop unrecognized --extension arg from remote --send invocation

ServerCli has no --extension field -- run_send_server already gets
fully-resolved --partial and --final-source from the client, so the
extension is never needed remotely. Passing it made every remote-source
pull fail with "unexpected argument '--extension' found".

src/client.rs

@@ -89,8 +89,6 @@ pub fn run(cli: ClientCli) -> Result<()> {
                 source_target.path().to_string(),
                 "--final-source".to_string(),
                 final_source_path.clone(),
-                "--extension".to_string(),
-                cli.extension.clone(),
                 "--chunk".to_string(),
                 cli.chunk.clone(),
                 "--level".to_string(),