chore: Fix up warnings from cargo +beta check. (#7453)

With upcoming release of 1.76 I did a check of current +beta (which
seems to already be at 1.77). These would cause CI pipeline failures
once 1.77 is out.

Release Notes:

- N/A

crates/gpui/src/scene.rs

@@ -863,6 +863,3 @@ impl PathVertex<Pixels> {
         }
     }
 }
-
-#[derive(Copy, Clone, Debug)]
-pub(crate) struct AtlasId(pub(crate) usize);