Remove add_sub_app api

[hymm]

Consider removing the add_sub_app api. This method is somewhat redundant with the insert_sub_app api. Consider:

let sub_app = App::empty();
app.add_sub_app(SubAppLabel, sub_app, extract_function);

vs

let sub_app = App::empty();
app.insert_sub_app(
  SubAppLabel,
  SubApp {
    app: sub_app,
    extract: Box::new(extract_function),
  }
);

This is a somewhat niche feature and may not deserve an extra api for slight better ergonomics.

See this comment from cart from the pipelined rendering pr

Maybe we should just remove this in favor of insert_sub_app. The insert semantics are "correct" anyway whereas add is generally used for repeatable / non-overwriting actions. This function is slightly more ergonomic, but SubApps aren't exactly a common pattern anyway.