fix: one leftover Arc::ptr_eq

[koivunej]

@knizhnik noticed that one instance of Arc::<dyn PersistentLayer>::ptr_eq was missed in #3558.

Now all ptr_eq which remain are in comments.

[koivunej]

Remaining are:

pageserver/src/tenant/layer_map.rs
301:            // this assertion is related to use of Arc::ptr_eq in Self::compare_arced_layers,
734:    /// Similar to `Arc::ptr_eq`, but only compares the object pointers, not vtables.
744:        // `Arc::ptr_eq` as of writing (at least up to 1.67) uses a comparison where both the

pageserver/src/tenant/layer_map/historic_layer_coverage.rs
436:    /// against some expectation. This allows to use `Arc::ptr_eq` or similar which would be

As I've checked a few times now. I never do remember search all in the first place, as I thought I had already dedupped the callsites when introducing compare_arced_layers.