rustc: Make unused_features lint warn by default

When it was un*known*_features it was reasonably to by deny by default.

cc #21798

src/librustc/lint/builtin.rs

@@ -1984,7 +1984,7 @@ declare_lint! {
 
 declare_lint! {
     pub UNUSED_FEATURES,
-    Deny,
+    Warn,
     "unused or unknown features found in crate-level #[feature] directives"
 }