Rollup merge of #44313 - RalfJung:book, r=nikomatsakis
Rollup merge of #44313 - RalfJung:book, r=nikomatsakis

rustbook: remove dead test functions

There is no "test" subcommand added to the `clap::App`, so this is all dead code.

Cc @steveklabnik -- your [commit](https://github.com/RalfJung/rust/commit/a076961fd0e3d8a68f8b047460b8f5191d203b08) introducing this stated the intention of having both commands, but it seems nobody has missed the `test` command since February.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust