This removes the port-specific definition of MP_PLAT_PRINT_STRN on the unix
port. Since fee7e5617f55b4778de74ee185fcc3950cdc7b6d this is no longer a
single function call so we are not really optimising anything over using
the default definition of MP_PLAT_PRINT_STRN which calls
mp_hal_stdout_tx_strn_cooked().