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

components/script/Cargo.toml

@@ -16,8 +16,7 @@ path = "lib.rs"
 debugmozjs = ['js/debugmozjs']
 profilemozjs = ['js/profilemozjs']
 unrooted_must_root_lint = ["script_plugins/unrooted_must_root_lint"]
-webidl_lint = ["script_plugins/webidl_lint"]
-default = ["unrooted_must_root_lint", "webidl_lint"]
+default = ["unrooted_must_root_lint"]
 webgl_backtrace = ["backtrace", "canvas_traits/webgl_backtrace"]
 js_backtrace = ["backtrace"]
 uwp = ["js/uwp"]

components/script_plugins/Cargo.toml

@@ -11,7 +11,3 @@ plugin = true
 
 [features]
 unrooted_must_root_lint = []
-webidl_lint = []
-
-[dependencies]
-weedle = "0.10"

components/script_plugins/lib.rs

@@ -24,18 +24,13 @@ extern crate rustc;
 extern crate rustc_driver;
 extern crate syntax;
 
-extern crate weedle;
-
 use rustc_driver::plugin::Registry;
 use syntax::feature_gate::AttributeType::Whitelisted;
 use syntax::symbol::Symbol;
 
 #[cfg(feature = "unrooted_must_root_lint")]
 mod unrooted_must_root;
 
-#[cfg(feature = "webidl_lint")]
-mod webidl_must_inherit;
-
 /// Utilities for writing plugins
 #[cfg(feature = "unrooted_must_root_lint")]
 mod utils;
@@ -49,15 +44,9 @@ pub fn plugin_registrar(reg: &mut Registry) {
         symbols.clone(),
     )));
 
-    #[cfg(feature = "webidl_lint")]
-    reg.register_late_lint_pass(Box::new(webidl_must_inherit::WebIdlPass::new(
-        symbols.clone(),
-    )));
-
     reg.register_attribute(symbols.allow_unrooted_interior, Whitelisted);
     reg.register_attribute(symbols.allow_unrooted_in_rc, Whitelisted);
     reg.register_attribute(symbols.must_root, Whitelisted);
-    reg.register_attribute(symbols.webidl, Whitelisted);
 }
 
 macro_rules! symbols {
@@ -82,7 +71,6 @@ symbols! {
     allow_unrooted_interior
     allow_unrooted_in_rc
     must_root
-    webidl
     alloc
     rc
     Rc