Remove extern "C" functions from wasm module

We don't need these functions and their export. They account for ~20kb in
the optimized .wasm - plus JS glue code.

sixtyfps_runtime/corelib/callbacks.rs

@@ -69,6 +69,7 @@ fn callback_simple_test() {
     assert_eq!(c.pressed.get(), true);
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub(crate) mod ffi {
     #![allow(unsafe_code)]