remove `#[deprecated]` from unstable & internal `SipHasher13` and `24` types
These types are unstable and `doc(hidden)` (under the internal feature `hashmap_internals`). Deprecating them only adds noise (`#[allow(deprecated)]`) to all places where they are used, so this PR removes the deprecation attributes from them.
It also includes a few other small cleanups in separate commits, including one I overlooked in rust-lang/rust#151228.