Remove .cargo/config (#421)

The `.cargo/config.toml` file has been supported since Rust 1.39, and is
only needed for testing, not for actually building the crate. There's
no harm in it being ignored.

This eliminates a warning caused by both `.cargo/config.toml` and
`.cargo/config` being present.

Signed-off-by: Joe Richey <joerichey@google.com>