Remove build script from serde_derive

The current serde_derive's MSRV is 1.56, and both underscore consts and
ptr::addr_of! are always available.

serde_derive/src/dummy.rs

@@ -1,23 +1,11 @@
-use proc_macro2::{Ident, TokenStream};
-use quote::format_ident;
+use proc_macro2::TokenStream;
 
 use syn;
 use try;
 
-pub fn wrap_in_const(
-    serde_path: Option<&syn::Path>,
-    trait_: &str,
-    ty: &Ident,
-    code: TokenStream,
-) -> TokenStream {
+pub fn wrap_in_const(serde_path: Option<&syn::Path>, code: TokenStream) -> TokenStream {
     let try_replacement = try::replacement();
 
-    let dummy_const = if cfg!(no_underscore_consts) {
-        format_ident!("_IMPL_{}_FOR_{}", trait_, unraw(ty))
-    } else {
-        format_ident!("_")
-    };
-
     let use_serde = match serde_path {
         Some(path) => quote! {
             use #path as _serde;
@@ -31,14 +19,10 @@ pub fn wrap_in_const(
     quote! {
         #[doc(hidden)]
         #[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
-        const #dummy_const: () = {
+        const _: () = {
             #use_serde
             #try_replacement
             #code
         };
     }
 }
-
-fn unraw(ident: &Ident) -> String {
-    ident.to_string().trim_start_matches("r#").to_owned()
-}