LibWeb: Remove unused should_invalidate_styles_on_attribute_changes()

This getter and setter were previously labelled as a "hack" and used to
disable style invalidation on attribute changes during the HTML parsing
phase (as it caused big sites's loading to be slow). These functions
are currently not used, so they can be removed:^)

Userland/Libraries/LibWeb/DOM/Document.h

@@ -100,9 +100,6 @@ public:
     String referrer() const;
     void set_referrer(String);
 
-    bool should_invalidate_styles_on_attribute_changes() const { return m_should_invalidate_styles_on_attribute_changes; }
-    void set_should_invalidate_styles_on_attribute_changes(bool b) { m_should_invalidate_styles_on_attribute_changes = b; }
-
     void set_url(const AK::URL& url) { m_url = url; }
     AK::URL url() const { return m_url; }
     AK::URL fallback_base_url() const;

Userland/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

@@ -128,7 +128,6 @@ HTMLParser::HTMLParser(DOM::Document& document, StringView input, String const&
 {
     m_tokenizer.set_parser({}, *this);
     m_document->set_parser({}, *this);
-    m_document->set_should_invalidate_styles_on_attribute_changes(false);
     auto standardized_encoding = TextCodec::get_standardized_encoding(encoding);
     VERIFY(standardized_encoding.has_value());
     m_document->set_encoding(standardized_encoding.value());
@@ -144,7 +143,6 @@ HTMLParser::HTMLParser(DOM::Document& document)
 
 HTMLParser::~HTMLParser()
 {
-    m_document->set_should_invalidate_styles_on_attribute_changes(true);
 }
 
 void HTMLParser::visit_edges(Cell::Visitor& visitor)