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

[dhebbeker]

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.