boards/ delete unused stm32f4discovery example

.ci/Jenkinsfile-compile

Makefile

boards/stm/32f4discovery/default.cmake

boards/stm/32f4discovery/firmware.prototype

boards/stm/32f4discovery/nuttx-config/include/board.h

boards/stm/32f4discovery/nuttx-config/nsh/defconfig

boards/stm/32f4discovery/nuttx-config/scripts/ld.script

boards/stm/32f4discovery/src/CMakeLists.txt

boards/stm/32f4discovery/src/board_config.h

boards/stm/32f4discovery/src/init.c

boards/stm/32f4discovery/src/led.c

boards/stm/32f4discovery/src/timer_config.c

boards/stm/32f4discovery/src/usb.c