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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