Remove executor::Null from the root public API

Using an `Application` with `executor::Null` does not make sense, as the
whole purpose of an `Application` is to allow executing async actions.

When async actions are not needed, `Sandbox` should be used instead.

src/executor.rs

@@ -1,5 +1,5 @@
 //! Choose your preferred executor to power your application.
-pub use crate::runtime::{executor::Null, Executor};
+pub use crate::runtime::Executor;
 
 pub use platform::Default;