LibWeb/CSS: Remove unused ParseError::InternalError

Author: https://github.com/AtkinsSJ
Commit: b7a108d7cb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5576

Libraries/LibWeb/CSS/Parser/Parser.h

@@ -149,9 +149,8 @@ public:
 private:
     Parser(ParsingParams const&, Vector<Token>);
 
-    enum class ParseError {
+    enum class ParseError : u8 {
         IncludesIgnoredVendorPrefix,
-        InternalError,
         SyntaxError,
     };
     template<typename T>