Fix unused lifetime relationships.

This code:

```rust
#[subenum(Dop)]
enum Dip<'a: 'b, 'b> {
    #[subenum(Dop)]
    A(&'a u8),
    B(&'b u8),
}
```

produces:

```rust
enum Dop<'a: 'b, 'b> {
    A(&'a u8),
}
```

which fails to compile.

We should prune the lifetimes that don't show up in any variants, even if other lifetimes depend on them.