LibGUI: Remove barely used AbstractView::is_highlighting_searching()

Author: https://github.com/d1823
Commit: aeffd9024e
Pull-request: https://github.com/SerenityOS/serenity/pull/9790
Reviewed-by: https://github.com/alimpfard ✅
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

Userland/Libraries/LibGUI/AbstractView.h

@@ -166,7 +166,6 @@ protected:
     void cancel_searching();
     void start_searching_timer();
     void do_search(String&&);
-    bool is_highlighting_searching(const ModelIndex&) const;
 
     ModelIndex drop_candidate_index() const { return m_drop_candidate_index; }