tests/public_api: Omit blanket impls

[Enselic]

CI is currently failing because

• we are a library without a versioned Cargo.lock,
• the blessed public API was made at a time when dependencies blanket implementations looked different.

Fix this by removing blanket implementations from the public API surface check. This should reduce maintenance because of lack of a Cargo.lock, at the cost if not fully checking the public API.

The "right" solution is might be to begin to version a Cargo.lock, so that CI is reproducible, but that seems more invasive than relaxing the public API check, so let's take the non-intrusive route and see how it plays out long term.