Remove out-of-date comment.

Keys and values now must be `Copy` due to erasing.

compiler/rustc_query_system/src/query/caches.rs

@@ -21,9 +21,6 @@ pub trait QueryCacheKey = Hash + Eq + Copy + Debug + for<'a> HashStable<StableHa
 /// Types implementing this trait are associated with actual key/value types
 /// by the `Cache` associated type of the `rustc_middle::query::Key` trait.
 pub trait QueryCache: Sized {
-    // `Key` and `Value` are `Copy` instead of `Clone` to ensure copying them stays cheap,
-    // but it isn't strictly necessary.
-    // FIXME: Is that comment still true?
     type Key: QueryCacheKey;
     type Value: Copy;