Clean up nonexistent/unused properties handling

Clean up nonexistent/unused properties handling

Never used since import and probably just ported over from NetBSD as-is;
"design-capacity" does not exist in the device tree binding.
"monitor-interval-ms" defaults to 250ms as per binding and could be used
in the sensor_task_register() call, but our framework only supports whole
seconds and there's no advantage over our current fixed poll interval of 5s.

OK patrick