LibWeb: Remove m_src_is_set field from HTMLScriptElement

Now that we pass an `old_value` parameter to `attribute_changed` it is
no longer necessary to store the current attribute state in
`HTMLScriptElement`.

Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h

@@ -135,8 +135,6 @@ private:
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer;
 
     size_t m_source_line_number { 1 };
-
-    bool m_src_is_set = false;
 };
 
 }