Remove `Predicate` implementations from our public API

[epage]

predicates is going to split into predicates and predicates-core. Only predicates-core can show up in our public API.

This work was done for our IntoOutputPredicate predicates but the IntoCodePredicates were missed.

See https://github.com/assert-rs/assert_cmd/blob/master/src/assert.rs#L370

[epage]

Predicates has now been split.

You can audit this issue by looking at the docs

• https://docs.rs/assert_cmd/0.9.0/assert_cmd/assert/trait.IntoCodePredicate.html
• https://docs.rs/assert_cmd/0.9.0/assert_cmd/assert/trait.IntoOutputPredicate.html

Does the type Predicate = ...; point to a type defined within the crate or from `predicates?