Auto merge of #24319 - servo:webidl-lint, r=jdm

Remove redundant webidl_must_inherit compiler plugin lint

At first I was considering moving it to a procedural macro (source-level information should be sufficient), and started by trying to reproduce the error case by changing `htmldivelement.rs` to use `Element` instead of `HTMLElement` as the first field.

The output was:

```rust
error[E0308]: mismatched types
   --> /home/simon/servo2/target/debug/build/script-4caa244faca7d10f/out/Bindings/HTMLDivElementBinding.rs:665:31
    |
665 |         let _: &HTMLElement = self.as_parent();
    |                               ^^^^^^^^^^^^^^^^ expected struct `dom::htmlelement::HTMLElement`, found struct `dom::element::Element`
    |
    = note: expected type `&dom::htmlelement::HTMLElement`
               found type `&dom::element::Element`
```

This line number is inside a generated method called `__assert_parent_type`. As far as I can tell, any case where this lint would error is already caught by such methods. The lint is therefore redundant and can safely be removed.

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