AP_HAL_Linux: UARTDriver remove unused field and tabs

libraries/AP_HAL_Linux/UARTDriver.h

@@ -83,7 +83,6 @@ private:
     void _deallocate_buffers();
 
     AP_HAL::OwnPtr<SerialDevice> _parseDevicePath(const char *arg);
-    uint64_t _last_write_time;
 
     // timestamp for receiving data on the UART, avoiding a lock
     uint64_t _receive_timestamp[2];