Remove spurious #[derive(Encode, Decode)] (#2759)

They did not compile, since `Encode` and `Decode` are deliberately not
implemented for `usize`.

substrate/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 89,
-	impl_version: 89,
+	impl_version: 90,
 	apis: RUNTIME_API_VERSIONS,
 };