stm: Remove long-obsolete stm/ port.

README.md

stm/.gitignore

stm/Makefile

stm/accel.c

stm/accel.h

stm/adc.c

stm/adc.h

stm/audio.c

stm/audio.h

stm/boards/NETDUINO_PLUS_2/mpconfigboard.h

stm/boards/NETDUINO_PLUS_2/pins.csv

stm/boards/PYBOARD3/mpconfigboard.h

stm/boards/PYBOARD3/pins.csv

stm/boards/PYBOARD4/mpconfigboard.h

stm/boards/PYBOARD4/pins.csv

stm/boards/STM32F4DISC/mpconfigboard.h

stm/boards/STM32F4DISC/pins.csv

stm/boards/make-pins.py

stm/boards/stm32f4xx-af.csv

stm/boards/stm32f4xx-prefix.c