board: drop support for PTTU platform

boards/pttu/Makefile

boards/pttu/Makefile.dep

boards/pttu/Makefile.features

boards/pttu/Makefile.include

boards/pttu/board_init.c

boards/pttu/include/board.h

boards/pttu/include/periph_conf.h

boards/pttu/tools/jtag.txt

boards/pttu/tools/olimex-arm-usb-ocd.cfg

boards/pttu/tools/openocd-pttu.cfg

boards/pttu/tools/openocd-pttu.sh

boards/pttu/tools/pttu_debug.gdb