Remove unused 'Export PDF' button in App.tsx

src/App.tsx

@@ -993,9 +993,6 @@ export default function App() {
                     </button>
                     {markdownPreview ? (
                       <>
-                        <button className="button" type="button">
-                          Export PDF
-                        </button>
                         <button className="button" type="button" onClick={handlePrintMarkdown}>
                           Print
                         </button>