remove use Cell in variance-object-types.rs

Author: dust1

src/test/ui/variance/variance-object-types.rs

@@ -1,6 +1,5 @@
 #![feature(rustc_attrs)]
 
-use std::cell::Cell;
 
 // For better or worse, associated types are invariant, and hence we
 // get an invariant result for `'a`.