fix(modem): Fixed clang-tidy warnings

components/esp_modem/include/cxx_include/esp_modem_cmux.hpp

@@ -142,7 +142,6 @@ private:
     size_t frame_header_offset;
     uint8_t *payload_start;
     size_t total_payload_size;
-    int instance;
     int sabm_ack;
 
     /**

components/esp_modem/include/cxx_include/esp_modem_netif.hpp

@@ -65,7 +65,6 @@ private:
     static void on_ppp_changed(void *arg, esp_event_base_t event_base, int32_t event_id, void *event_data);
 
     std::shared_ptr<DTE> ppp_dte;
-    esp_netif_t *netif;
     struct ppp_netif_driver driver {};
     SignalGroup signal;
     static const size_t PPP_STARTED = SignalGroup::bit0;