Clean up previous Cargo.lock files

[Scrumplex]

Assuming the following nvfetcher.toml:

[foo]
src.git = "https://github.com/foo/bar.git"
fetch.git = "https://github.com/foo/bar.git"
cargo_locks = ["Cargo.lock"]

Now, if I run nvfetcher --commit-changes it'll create a new Cargo.lock at _source/foo-*/Cargo.lock without cleaning up the old one if there is a new commit.

I'd like nvfetcher to get rid of old (and now unused) lock files to keep Git diffs small