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/build.rs

@@ -1,38 +0,0 @@
-use std::env;
-use std::process::Command;
-use std::str;
-
-// The rustc-cfg strings below are *not* public API. Please let us know by
-// opening a GitHub issue if your build environment requires some way to enable
-// these cfgs other than by executing our build script.
-fn main() {
-    println!("cargo:rerun-if-changed=build.rs");
-
-    let minor = match rustc_minor_version() {
-        Some(minor) => minor,
-        None => return,
-    };
-
-    // Underscore const names stabilized in Rust 1.37:
-    // https://blog.rust-lang.org/2019/08/15/Rust-1.37.0.html#using-unnamed-const-items-for-macros
-    if minor < 37 {
-        println!("cargo:rustc-cfg=no_underscore_consts");
-    }
-
-    // The ptr::addr_of! macro stabilized in Rust 1.51:
-    // https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#stabilized-apis
-    if minor < 51 {
-        println!("cargo:rustc-cfg=no_ptr_addr_of");
-    }
-}
-
-fn rustc_minor_version() -> Option<u32> {
-    let rustc = env::var_os("RUSTC")?;
-    let output = Command::new(rustc).arg("--version").output().ok()?;
-    let version = str::from_utf8(&output.stdout).ok()?;
-    let mut pieces = version.split('.');
-    if pieces.next() != Some("rustc 1") {
-        return None;
-    }
-    pieces.next()?.parse().ok()
-}