unix: Remove custom definition of MP_PLAT_PRINT_STRN.
unix: Remove custom definition of MP_PLAT_PRINT_STRN.

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().

pramasoul/micropython,bvernoux/micropython,MrSurly/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,kerneltask/micropython,tobbad/micropython,bvernoux/micropython,pozetroninc/micropython,MrSurly/micropython,adafruit/circuitpython,henriknelson/micropython,kerneltask/micropython,tobbad/micropython,pramasoul/micropython,adafruit/circuitpython,bvernoux/micropython,pozetroninc/micropython,henriknelson/micropython,tobbad/micropython,bvernoux/micropython,pramasoul/micropython,kerneltask/micropython,pozetroninc/micropython,selste/micropython,selste/micropython,henriknelson/micropython,tobbad/micropython,selste/micropython,tobbad/micropython,MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,pramasoul/micropython,kerneltask/micropython,henriknelson/micropython,selste/micropython,adafruit/circuitpython,MrSurly/micropython,pramasoul/micropython,adafruit/circuitpython,pozetroninc/micropython,selste/micropython,kerneltask/micropython,pozetroninc/micropython