rtic-sync: Remove unstable flag, and add defmt derives (#889)

xtask/src/argument_parsing.rs

@@ -90,7 +90,6 @@ impl Package {
                     .chain(std::iter::once(None))
                     .collect()
             }
-            Package::RticSync => vec![Some("unstable".to_string()), None],
             _ => vec![None],
         }
     }