Remove `nll` from `rust_2018_preview`

NLL isn't quite ready yet so gonna hold off on inserting it into the preview.

src/libsyntax/feature_gate.rs

@@ -190,7 +190,7 @@ pub fn walk_feature_fields<F>(&self, mut f: F)
     (active, rustc_attrs, "1.0.0", Some(29642), None),
 
     // Allows the use of non lexical lifetimes; RFC 2094
-    (active, nll, "1.0.0", Some(43234), Some(Edition::Edition2018)),
+    (active, nll, "1.0.0", Some(43234), None),
 
     // Allows the use of #[allow_internal_unstable]. This is an
     // attribute on macro_rules! and can't use the attribute handling