Tidy doesn't check for unused extern crates

[Mark-Simulacrum]

Since 44a01b8, tidy no longer emits unused crate warnings -- i.e., Cargo.toml has a dependency but the associated library does not. Due to use of 2018 edition implicit imports, this is no longer an easy thing to do (any file within the crate can use the crate, and extern crate isn't required).

I'm thinking we can probably just remove this tidy lint for now in favor of something like #57274 in the future.