Try to have time_ns same as std

Author: ShaharNaveh

vm/src/stdlib/time.rs

@@ -121,18 +121,11 @@ mod decl {
         Ok(())
     }
 
-    #[cfg(not(target_os = "wasi"))]
     #[pyfunction]
     fn time_ns(vm: &VirtualMachine) -> PyResult<u64> {
         Ok(duration_since_system_now(vm)?.as_nanos() as u64)
     }
 
-    #[cfg(target_os = "wasi")]
-    #[pyfunction]
-    fn time_ns(_vm: &VirtualMachine) -> PyResult<u64> {
-        unimplemented!("time.time_ns is not implemented yet for wasi");
-    }
-
     #[pyfunction]
     pub fn time(vm: &VirtualMachine) -> PyResult<f64> {
         _time(vm)