book: Remove suggestions to add 'extern crate' imports.
book: Remove suggestions to add 'extern crate' imports.

Gir generates edition = "2018" crates now, which do not require them:
https://doc.rust-lang.org/edition-guide/rust-2018/path-changes.html

gtk-rs/gir,GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir