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/Selector.cpp

@@ -25,10 +25,6 @@ Selector::Selector(Vector<CompoundSelector>&& compound_selectors)
     }
 }
 
-Selector::~Selector()
-{
-}
-
 // https://www.w3.org/TR/selectors-4/#specificity-rules
 u32 Selector::specificity() const
 {