cmake: Omit if statement that is always true
cmake: Omit if statement that is always true

We had an if-statement around policy 79 because we used to need to
support CMake versions before 3.13.0. But now 3.13.1 is the minimum
required version so the if statement always evaluates to true and can
be omitted.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

ldts/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr