Everywhere: Fix trivial -Wunnecessary-virtual-specifier instances

- `Threading::Thread` is not polymorphic, there is no need for a virtual
  destructor.
- `HTMLAnchorElement::has_download_preference` isn't overridden by
  anything.

This warning was introduced in llvm/llvm-project#131188.

Libraries/LibThreading/Thread.h

@@ -55,7 +55,7 @@ public:
         return adopt_nonnull_ref_or_enomem(new (nothrow) Thread(move(action), thread_name));
     }
 
-    virtual ~Thread();
+    ~Thread();
 
     ErrorOr<void> set_priority(int priority);
     ErrorOr<int> get_priority() const;

Libraries/LibWeb/HTML/HTMLAnchorElement.h

@@ -38,12 +38,14 @@ public:
 private:
     HTMLAnchorElement(DOM::Document&, DOM::QualifiedName);
 
+    bool has_download_preference() const;
+
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
+    // ^DOM::EventTarget
     virtual bool has_activation_behavior() const override;
     virtual void activation_behavior(Web::DOM::Event const&) override;
-    virtual bool has_download_preference() const;
 
     // ^DOM::Element
     virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_) override;