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.

components/dom_struct/lib.rs

@@ -19,7 +19,6 @@ pub fn dom_struct(args: TokenStream, input: TokenStream) -> TokenStream {
         #[derive(DenyPublicFields, DomObject, JSTraceable, MallocSizeOf)]
         #[must_root]
         #[repr(C)]
-        #[webidl]
     };
 
     // Work around https://github.com/rust-lang/rust/issues/46489