Drop `lightning`-specific code in `lightning::crypto`

Drop `lightning`-specific code in `lightning::crypto`

In a few commits, we'll reuse the `chacha20.rs` and `poly1305.rs`
files in a new crate (via symlink). In order to do so, we need to
first avoid any references to things in the `lightning` crate,
which we do here by dropping references to `crate::prelude`.