zephyr/main: always call sys_clock_disable()

Build-time conditional call is not needed anymore since the empty
sys_clock_disable() callback is provided if the platform
doesn't support system clock disable capability.

Reference:
https://github.com/zephyrproject-rtos/zephyr/commit/78dc8ce33828b1d995d520751fe761ade6103155



Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>