Remove spurious by-ref argument to destructors

Destructors were internally declared with an extra (hidden) nil-typed
argument that was passed in by-ref mode. This was causing spurious
mode warnings. Deleted it. Also some misc. cleanup because I
couldn't help myself.

src/libsyntax/ast_util.rs

@@ -398,10 +398,8 @@ fn operator_prec(op: ast::binop) -> uint {
 
 fn dtor_dec() -> fn_decl {
     let nil_t = @{id: 0, node: ty_nil, span: dummy_sp()};
-    // dtor has one argument, of type ()
-    {inputs: ~[{mode: ast::expl(ast::by_ref),
-                ty: nil_t, ident: parse::token::special_idents::underscore,
-                id: 0}],
+    // dtor has no args
+    {inputs: ~[],
      output: nil_t, cf: return_val}
 }