LibMarkdown: Remove unused Paragraph::add_line() declaration

Author: https://github.com/linusg
Commit: 8fe7983cc5
Pull-request: https://github.com/SerenityOS/serenity/pull/5961

Userland/Libraries/LibMarkdown/Paragraph.h

@@ -59,8 +59,6 @@ public:
     virtual String render_to_html() const override;
     virtual String render_for_terminal(size_t view_width = 0) const override;
 
-    void add_line(NonnullOwnPtr<Line>&& line);
-
 private:
     NonnullOwnPtrVector<Line> m_lines;
 };