The sys.stdin.buffer and sys.stdout.buffer streams work just as well (and
are just as fast) as pyb.USB_VCP on stm32 devices, so there's no need to
have the USB_VCP specialisation code, which just adds complexity.
Also, on stm32 devices with both USB and UART (or other serial interface),
if something other than the USB_VCP port is used for the serial connection
then mpremote mount will not work because it will default to reading and
writing on USB_VCP instead of the other connected serial stream.
As part of this simplification, support for a second port as input is
removed (this feature was never exposed to the user).
Signed-off-by: Damien George <damien@micropython.org>
226c0341ca