use `std::cout` instead of `serial_port::cout`

By accident I used `std::cout` in the simulator. It did write to the (simulated) serial port.

If this works as expected, we do not need any more:

- `SerialOutputStreamBuffer`
- `serial_port::cout`
- ... and more?

This would reduce our code base, which is good, as we have to maintain less code.