(uart, debug) first part of fixes after code review

- remove `uart_status` from eu7_application class
- remove copy&pasted `m_scenariopanel` size settings
- remove unnecessary code `debug_panel::render_section_uart()`
- remove #ifdef from `driveruipanels.h` textlines
- set `data_pending` to false when establishing a new connection

application.h

@@ -94,9 +94,6 @@ public:
         is_server() const;
     bool
         is_client() const;
-#ifdef WITH_UART
-    UartStatus uart_status;
-#endif
 
 private:
 // types