Remove use_extern_macros features

This has now been stabilized!

examples/webaudio/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(use_extern_macros, nll)]
+#![feature(nll)]
 
 extern crate wasm_bindgen;
 extern crate web_sys;
@@ -144,4 +144,4 @@ impl FmOsc {
     }
 
 
-}
+}