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/DOMStringMap.cpp

@@ -15,9 +15,7 @@ DOMStringMap::DOMStringMap(DOM::Element& associated_element)
 {
 }
 
-DOMStringMap::~DOMStringMap()
-{
-}
+DOMStringMap::~DOMStringMap() = default;
 
 // https://html.spec.whatwg.org/multipage/dom.html#concept-domstringmap-pairs
 Vector<DOMStringMap::NameValuePair> DOMStringMap::get_name_value_pairs() const