Remove (now) unused variables from retain_back unit test

Author: nullstalgia

src/deque.rs

@@ -2562,18 +2562,12 @@ mod tests {
 
             assert_eq!(Droppable::count(), LEN as i32);
 
-            let (front, back) = tester.as_slices();
-
             tester.retain_back(TRUNC);
             assert_eq!(tester.len(), TRUNC);
             assert_eq!(Droppable::count(), TRUNC as i32);
 
-            let (front, back) = tester.as_slices();
-
             tester.retain_back(0);
 
-            let (front, back) = tester.as_slices();
-
             assert_eq!(tester.len(), 0);
             assert_eq!(Droppable::count(), 0);
         }