Remove last traces of auth keyword

The reference now has an empty hole where the auth keyword used to be.
Changing the keyword table seems to require manually sorting the
keywords and putting them back into some kind of arcane interleaved
order. I'll open an issue to actually fix this.

Closes #1211

src/comp/syntax/fold.rs

@@ -168,7 +168,6 @@ fn noop_fold_crate_directive(cd: crate_directive_, fld: ast_fold) ->
           }
           cdir_view_item(vi) { cdir_view_item(fld.fold_view_item(vi)) }
           cdir_syntax(_) { cd }
-          cdir_auth(_, _) { cd }
         }
 }