LibWeb: Fix a bunch of trivial clang-tidy warnings in StyleComputer

- Replace "auto" with "auto const" where appropriate.
- Remove an unused struct.
- Make sort_matching_rules() a file-local static function.
- Remove some unnecessary includes.

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -65,7 +65,6 @@ public:
     };
 
     Vector<MatchingRule> collect_matching_rules(DOM::Element const&, CascadeOrigin = CascadeOrigin::Any) const;
-    void sort_matching_rules(Vector<MatchingRule>&) const;
     struct CustomPropertyResolutionTuple {
         Optional<StyleProperty> style {};
         u32 specificity { 0 };