LibWeb: Remove now-unused null values from Selector enums

Now that we use a Variant for the SimpleSelector's data, we don't need
to instantiate empty structs or variables for the types that aren't
used, and so we can remove `PseudoElement::None`,
`PsuedoClass::Type::None` and `Attribute::MatchType::None`.

Also, we now always initialize a SimpleSelector with a type, so
`SimpleSelector::Type::Invalid` can go too.

Userland/Libraries/LibWeb/CSS/Selector.cpp

@@ -105,8 +105,6 @@ u32 Selector::specificity() const
             case SimpleSelector::Type::Universal:
                 // ignore the universal selector
                 break;
-            case SimpleSelector::Type::Invalid:
-                break;
             }
         }
     }