refactor(backend): remove unused err variable in auth middleware

backend/middleware/auth.js

@@ -14,7 +14,7 @@ module.exports = function (req, res, next) {
     const decoded = jwt.verify(token, process.env.JWT_SECRET);
     req.user = decoded.user;
     next();
-  } catch (err) {
+  } catch {
     res.status(401).json({ msg: "Token is not valid" });
   }
 };