add allow(dead_code) to substrate weight templates (#7408)

address failed CI after full regeneration

Example https://github.com/paritytech/polkadot-sdk/pull/7406
Failed CI
https://github.com/paritytech/polkadot-sdk/actions/runs/13070646240

Monkey-patched weights which have been overridden by automation

![image](https://github.com/user-attachments/assets/ecf69173-f4dd-4113-a319-4f29d779ecae)

substrate/.maintain/frame-umbrella-weight-template.hbs

@@ -16,6 +16,7 @@
 #![allow(unused_parens)]
 #![allow(unused_imports)]
 #![allow(missing_docs)]
+#[allow(dead_code)]
 
 use frame::weights_prelude::*;
 

substrate/.maintain/frame-weight-template.hbs

@@ -16,6 +16,7 @@
 #![allow(unused_parens)]
 #![allow(unused_imports)]
 #![allow(missing_docs)]
+#[allow(dead_code)]
 
 use frame_support::{traits::Get, weights::{Weight, constants::RocksDbWeight}};
 use core::marker::PhantomData;