Libraries: Use default constructors/destructors in LibWeb

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibWeb/HTML/HTMLPreElement.cpp

@@ -13,8 +13,6 @@ HTMLPreElement::HTMLPreElement(DOM::Document& document, DOM::QualifiedName quali
 {
 }
 
-HTMLPreElement::~HTMLPreElement()
-{
-}
+HTMLPreElement::~HTMLPreElement() = default;
 
 }