Auto merge of #5176 - alexcrichton:rustfmt, r=alexcrichton

Add `cargo fmt` to CI and delete `rustfmt.toml`

This commit adds CI to run `cargo fmt` over Cargo itself as well as the internal
`crates-io` crate. This should switch Cargo to the "default style" (aka whatever
rustfmt spits out) and ensure that we keep it that way via CI. Hopefully this
won't be too much of a bother to keep up and running in CI as it should just be
a `cargo fmt` away!