🛠️ kill `Predicate::Equate`, which is dead code

[nikomatsakis]

Just to clean things up, we should remove Predicate::Equate, which is not used presently. I did this once, in the following commit, so you could start by cherry-picking it (or just use it as a guide):

nikomatsakis@6cd24a5

[nikomatsakis]

cc @rust-lang/wg-traits -- this is a simple refactoring, if you'd like to get your hands dirty and do a bit of a "dry run" on the PR authoring process. Feel free to ping me with questions, and you can put r? @nikomatsakis in the PR when you open it. =)