Remove unneeded extern crate serde

[dtolnay]

Since rust-lang/rust#42588, Rust warns by default on unused extern crates. The readme example code, as well as possibly other examples in rustdoc, has extern crate serde but does not refer to anything from it. It is there as a reminder that the generated code requires serde to be listed as a dependency in Cargo.toml.

I imagine we can make up for it by providing an explicit Cargo.toml snippet instead.

[dependencies]
serde = "1.0"
serde_derive = "1.0"
serde_json = "1.0"

[dtolnay]

@ishitatsuyuki I don't plan to fix this in Serde docs. I would prefer to consider this a false positive and fix it in Rust instead. rust-lang/rust#44294