Remove allow(unused_variables) for rustc_transmute.

This was hiding some genuine sins, including unused arguments in
numerous functions/methods (incl. trait methods), and some unnecessary
computation.

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -617,7 +617,6 @@ where
             )?;
 
             let certainty = ecx.is_transmutable(
-                goal.param_env,
                 goal.predicate.trait_ref.args.type_at(0),
                 goal.predicate.trait_ref.args.type_at(1),
                 assume,