Remove unused libc dependency from Cargo.toml

[metent]

Thanks for contributing to chrono!

• Have you added yourself and the change to the changelog? (Don't worry
  about adding the PR number)
• If this pull request fixes a bug, does it add a test that verifies that
  we can't reintroduce it?

libc dependency should be removed as it is no longer directly used by chrono. I'm not sure whether the changelog should document this change.

[esheppa]

Hi @metent - thanks for this, it looks good to me. If possible could you update the documentation about this feature here. I think its worth putting in the changelog as this could be "potentially breaking" in a weird case - because libc is an optional crate, people could add it to their chrono features. We could resolve this by adding a feature called "libc" that does nothing as specified here

[metent]

Thanks for the pointer. Should be good now.

[esheppa]

Thanks @metent