clippy: Fix last few warnings (#32270)

* Fix clippy in components/script

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do (components/script/dom/htmlformelement.rs:896:20)

warning: `Box::new(_)` of default value (components/script/dom/paintworkletglobalscope.rs:291:29)

warning: this creates an owned instance just for comparison (components/script/dom/radionodelist.rs:105:50)

* Fix clippy in layout_thread (2013 and 2020)

warning: this `if` statement can be collapsed (components/layout_thread/lib.rs:876:17)

warning: the following explicit lifetimes could be elided: 'a (components/layout_thread/lib.rs:239 and 2020 same line)

warning: deref which would be done by auto-deref (components/layout_thread/lib.rs:500 and 1289)

warning: dereferencing a tuple pattern where every element takes a reference (components/layout_thread/lib.rs:503,1562 and 2020 line 1153)

warning: useless conversion to the same type: `style::invalidation::element::restyle_hints::RestyleHint` (components/layout_thread_2020/lib.rs:742:36)

* Fix clippy in components/servo

warning: constants have by default a `'static` lifetime (components/servo/lib.rs:1238:31)

warning: creating a `let` binding to a value of unit type, which usually
can't be used afterwards (5 occurances in components/servo/lib.rs)

* FIx clippy in ports/servoshell

warning: this expression creates a reference which is immediately dereferenced by the compiler (ports/servoshell/app.rs:251:89)

warning: using `clone` on type `Option<TopLevelBrowsingContextId>` which implements the `Copy` trait (ports/servoshell/webview.rs:122:9)

components/script/dom/htmlformelement.rs

@@ -893,7 +893,7 @@ impl HTMLFormElement {
     // https://html.spec.whatwg.org/multipage/#submit-body
     fn submit_entity_body(
         &self,
-        form_data: &mut Vec<FormDatum>,
+        form_data: &mut [FormDatum],
         mut load_data: LoadData,
         enctype: FormEncType,
         encoding: &'static Encoding,