This removes the port-specific definition of MP_PLAT_PRINT_STRN on the
windows port, so that the default mp_hal_stdout_tx_strn_cooked() is always
used. This fixes releasing the GIL during the call to write() (this was
missed in bc3499f0103abcae39ad048bb42a518a665b8497).
Also, mp_hal_dupterm_tx_strn() was defined but never used anywhere so it is
safe to delete it.