net/netlink/af_netlink.h: Remove unused structure.

#define NLGRPSZ(x)	(ALIGN(x, sizeof(unsigned long) * 8) / 8)

#define NLGRPLONGS(x)	(NLGRPSZ(x)/sizeof(unsigned long))

struct netlink_ring {

	void			**pg_vec;

	unsigned int		head;

	unsigned int		frames_per_block;

	unsigned int		frame_size;

	unsigned int		frame_max;

	unsigned int		pg_vec_order;

	unsigned int		pg_vec_pages;

	unsigned int		pg_vec_len;

	atomic_t		pending;

};

struct netlink_sock {

	/* struct sock has to be the first member of netlink_sock */

	struct sock		sk;