allow dead code annotations

Summary: some new dead code annotations thrown up by 1.78.0

Reviewed By: JakobDegen

Differential Revision: D57165902

fbshipit-source-id: 598e726f1d4afc955b44ea4f7e3974f58f1299f6

Author: shayne-fletcher

gazebo/src/cell.rs

@@ -208,7 +208,6 @@ impl<T: ?Sized> AsARef<T> for RefCell<T> {
 
 #[cfg(test)]
 mod tests {
-    use std::cell::RefCell;
     use std::mem;
 
     use super::*;