Delete unused weak_handle INSTANCE_INDEX_SHADER_HANDLE. (#18507)

# Objective

- This variable is unused and never populated. I searched for the
literal text of the const and got no hits.

## Solution

- Delete it!

## Testing

- None.

crates/bevy_render/src/lib.rs

@@ -285,8 +285,6 @@ struct FutureRenderResources(Arc<Mutex<Option<RenderResources>>>);
 #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, AppLabel)]
 pub struct RenderApp;
 
-pub const INSTANCE_INDEX_SHADER_HANDLE: Handle<Shader> =
-    weak_handle!("475c76aa-4afd-4a6b-9878-1fc1e2f41216");
 pub const MATHS_SHADER_HANDLE: Handle<Shader> =
     weak_handle!("d94d70d4-746d-49c4-bfc3-27d63f2acda0");
 pub const COLOR_OPERATIONS_SHADER_HANDLE: Handle<Shader> =