rustc: remove the rest of drop

Removes:

ast::struct_def::dtor
syntax::ast::ii_dtor
syntax::visit::fk_dtor
syntax::ast_map::node_dtor
syntax:struct_dtor

src/libsyntax/ext/pipes/pipec.rs

@@ -431,7 +431,6 @@ impl gen_init for protocol {
             dummy_sp(),
             ast::struct_def {
                 fields: fields,
-                dtor: None,
                 ctor_id: None
             },
             cx.strip_bounds(&generics))