scripts: gen_priv_stacks: remove unused variable
scripts: gen_priv_stacks: remove unused variable

Variable priv_stack_decl_size is not used by the script,
so we can remove it.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

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