boards: use default value for XTIMER_DEV and _CHAN

boards/airfy-beacon/include/board.h

boards/b-l072z-lrwan1/include/board.h

boards/calliope-mini/include/board.h

boards/chronos/include/board.h

boards/microbit/include/board.h

boards/msb-430-common/include/board_common.h

boards/nrf51dongle/include/board.h

boards/nucleo-f401/include/board.h

boards/nucleo-f410/include/board.h

boards/nucleo-f411/include/board.h

boards/nucleo-f446/include/board.h

boards/nucleo-l053/include/board.h

boards/nucleo-l073/include/board.h

boards/nucleo-l152/include/board.h

boards/nucleo144-f303/include/board.h

boards/nucleo144-f412/include/board.h

boards/nucleo144-f413/include/board.h

boards/nucleo144-f429/include/board.h

boards/nucleo144-f446/include/board.h

boards/pca10000/include/board.h