maint: Remove RUST_NIX_C_RAW_EXTRA_CFLAGS

The standard bindgen flag BINDGEN_EXTRA_CLANG_ARGS seems to do the
same thing.

(cherry picked from commit 599eece08bee28d72c4ec20c8578428c158a19c1)

flake.nix

@@ -47,8 +47,6 @@
             buildInputs = [
               config.packages.nix
             ];
-            # Workaround: the gcc in the devshell doesn't find libc headers
-            RUST_NIX_C_RAW_EXTRA_CFLAGS = "-I${pkgs.stdenv.cc.libc.dev}/include";
             nativeBuildInputs = [
               pkgs.rust-analyzer
               pkgs.nixpkgs-fmt

rust/nix-c-raw/build.rs

@@ -45,12 +45,6 @@ fn c_headers() -> Vec<String> {
         args.push(format!("-I{}", path.to_str().unwrap()));
     }
 
-    if let Ok(cflags) = std::env::var("RUST_NIX_C_RAW_EXTRA_CFLAGS") {
-        for flag in cflags.split_whitespace() {
-            args.push(flag.to_string());
-        }
-    }
-
     // write to stderr for debugging
     eprintln!("c_headers: {:?}", args);
     args