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/StyleValue.cpp

@@ -23,10 +23,6 @@ StyleValue::StyleValue(Type type)
 {
 }
 
-StyleValue::~StyleValue()
-{
-}
-
 AngleStyleValue const& StyleValue::as_angle() const
 {
     VERIFY(is_angle());