bdev: remove unused child_bdevs fields

	/** Represents maximum unmap block descriptor count */

	uint32_t max_unmap_bdesc_count;

	/** array of child block dev that is underneath of the current dev */

	struct spdk_bdev **child_bdevs;

	/** number of child blockdevs allocated */

	int num_child_bdevs;

	/** generation value used by block device reset */

	uint32_t gencnt;