fix some lints

Author: nikomatsakis

crates/formality-prove/src/prove/minimize/test.rs

@@ -34,7 +34,7 @@ fn minimize_a() {
     "#]]
     .assert_debug_eq(&(&env, &term));
 
-    let (mut env_min, mut term_min, m) = minimize(env, term);
+    let (mut env_min, term_min, m) = minimize(env, term);
 
     expect![[r#"
         (

crates/formality-prove/src/prove/prove_normalize.rs

@@ -1,6 +1,5 @@
 use formality_types::{
-    cast::{Downcast, DowncastFrom},
-    derive_links::DowncastTo,
+    cast::Downcast,
     grammar::{
         AliasTy, AtomicRelation, InferenceVar, Parameter, RigidTy, TyData, Variable, Wc, WcData,
         Wcs,