fix(lib): Remove unused unimplemented! macro.

[MJDSys]

This macro isn't used anywhere, std now has an unimplemented macro
if we want to use it, and the nightly compiler now warns that this
unused. This warning is a failure when compiling tests.

• The commit messages match the guidelines in https://github.com/hyperium/hyper/blob/master/CONTRIBUTING.md#git-commit-guidelines

[seanmonstar]

Thanks!

I technically had this in hyper's source so that I could add messages to the macro invocations, like unimplemented!("Conn::poll_complete"). I found that immensely useful, and so I've filed a PR to add that functionality to rust proper: rust-lang/rust#42155