Remove usage of wasm_import_module feature

This is now stabilized! Also tweak usage of it to the stable version.

crates/web-sys/src/lib.rs

@@ -1,4 +1,3 @@
-#![feature(wasm_import_module)]
 #![doc(html_root_url = "https://docs.rs/web-sys/0.2")]
 
 extern crate wasm_bindgen;

crates/web-sys/tests/all/headers.rs

@@ -6,7 +6,7 @@ fn headers() {
         .file(
             "src/lib.rs",
             r#"
-                #![feature(use_extern_macros, wasm_import_module)]
+                #![feature(use_extern_macros)]
                 extern crate wasm_bindgen;
                 use wasm_bindgen::prelude::*;
                 extern crate web_sys;