LibWeb: Unbreak character reference and DOCTYPE parsing post-UTF-8

Oops, these were still using the byte-offset cursor. My goodness is it
unergonomic to index into UTF-8 strings, but Dr. Bugaev says it's good.

There is lots of room for improvement here. Just like the rest of the
tokenizer and parser. We'll have to do a few optimization passes over
them once they mature.

Libraries/LibWeb/Parser/HTMLTokenizer.h

@@ -169,7 +169,6 @@ private:
     String m_decoded_input;
 
     StringView m_input;
-    size_t m_cursor { 0 };
 
     Utf8View m_utf8_view;
     AK::Utf8CodepointIterator m_utf8_iterator;