screencast: remove check on loop_iterate

[columbarius]

According to the documentation [1] pw_loop_iterate (a macro for spa_loop_control_iterate) will return the number of dispatched fd's which don't have any value for error handling.

[1] https://docs.pipewire.org/group__spa__loop.html#ga3bae0b32100f5752b3372a505c8e04f6

[emersion]

We should still check for errors with if (pw_loop_iterate(…) < 0) and ideally break out of the loop on error.