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/CSS/CSSStyleDeclaration.cpp

@@ -16,14 +16,6 @@ PropertyOwningCSSStyleDeclaration::PropertyOwningCSSStyleDeclaration(Vector<Styl
 {
 }
 
-PropertyOwningCSSStyleDeclaration::~PropertyOwningCSSStyleDeclaration()
-{
-}
-
-CSSStyleDeclaration::~CSSStyleDeclaration()
-{
-}
-
 String PropertyOwningCSSStyleDeclaration::item(size_t index) const
 {
     if (index >= m_properties.size())
@@ -43,10 +35,6 @@ ElementInlineCSSStyleDeclaration::ElementInlineCSSStyleDeclaration(DOM::Element&
 {
 }
 
-ElementInlineCSSStyleDeclaration::~ElementInlineCSSStyleDeclaration()
-{
-}
-
 size_t PropertyOwningCSSStyleDeclaration::length() const
 {
     return m_properties.size();