clean unused use

nac3core/src/typecheck/typedef/mod.rs

@@ -1,5 +1,5 @@
 use itertools::{chain, zip, Itertools};
-use std::borrow::{Borrow, Cow};
+use std::borrow::Cow;
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::iter::once;