kernel: poll: remove unreachable code
kernel: poll: remove unreachable code

register_event always returns 0, so the conditional will
always take the first branch and code in the else part
is never reached.

Fixes #31282

Signed-off-by: Ningx Zhao <b27bf4706dc5a221d2ce626ff9b89d8daf27fcdd@intel.com>

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