Delete obsolete .cargo/config.toml (#979)

Maturin adds these flags by itself, but the presence of this file can
seemingly cause them to not get added at all in some environments,
likely related to them using their own `.cargo/config.toml` file.

This was possibly caused by
<PyO3/maturin@74baa67>,
as we noticed it when bumping Maturin past that commit in
Nixpkgs. Removing the file fixes the macOS build in our environment.

Author: emilazy