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/ser.rs

@@ -59,8 +59,6 @@ pub fn expand_derive_serialize(
 
     Ok(dummy::wrap_in_const(
         cont.attrs.custom_serde_path(),
-        "SERIALIZE",
-        ident,
         impl_block,
     ))
 }