boards: stm32: Remove redundant CORTEX_M_SYSTICK=y

CORTEX_M_SYSTICK is enabled by default on all STM32 based targets,
in common soc Kconfig files.
Forcing its definition in board files is redundant and prevents
to disable it when activating LPTIM as ticker.
Remove these definitions

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>