simplify check on flush.pug

Signed-off-by: eternal-flame-AD <yume@yumechi.jp>

packages/backend/src/server/web/views/flush.pug

@@ -6,7 +6,7 @@ html
 		const msg = document.getElementById('msg');
 		const successText = `\nSuccess Flush! <a href="/">Back to Misskey</a>\n成功しました。<a href="/">Misskeyを開き直してください。</a>`;
 
-		if (!document.cookie.includes('http-flush-failed=1')) {
+		if (!document.cookie) {
 			message('Your site data is fully cleared by your browser.');
 			message(successText);
 		} else {