src: remove user_data from TimerWrap

There’s no point in having an opaque user data pointer when we’re
already using `std::function`.

PR-URL: https://github.com/nodejs/node/pull/34252


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: David Carlier <devnexen@gmail.com>