wasi-nn: pare down build.rs

The `build.rs` script was only telling Wiggle where the WITX files
lived. We do need to retain it, though, for `env!("OUT_DIR")` to still
work.

prtest:full

crates/wasi-nn/build.rs

@@ -1,13 +1,4 @@
-//! This build script:
-//!  - has the configuration necessary for the Wiggle and WITX macros.
 fn main() {
-    // This is necessary for Wiggle/WITX macros.
-    let cwd = std::env::current_dir().unwrap();
-    let wasi_root = cwd.join("witx");
-    println!("cargo:rustc-env=WASI_ROOT={}", wasi_root.display());
-
-    // Also automatically rebuild if the WITX files change
-    for entry in walkdir::WalkDir::new(wasi_root) {
-        println!("cargo:rerun-if-changed={}", entry.unwrap().path().display());
-    }
+    // No build step needed: this just ensures `env!("OUT_DIR")` is for test
+    // artifacts.
 }