7797: Format generated lints and features manually r=matklad a=lnicola
As `quote` and `rustfmt` leave them on a single line, which makes running `grep` in the repository quite annoying.
Also removes a dead `gen_features.rs` file (`gen_lint_completions.rs` does the same thing).
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>