backing-dev: Kill set but not used var in bdi_debug_stats_show()

	unsigned long background_thresh;

	unsigned long dirty_thresh;

	unsigned long bdi_thresh;

	unsigned long nr_dirty, nr_io, nr_more_io, nr_wb;

	unsigned long nr_dirty, nr_io, nr_more_io;

	struct inode *inode;

	nr_wb = nr_dirty = nr_io = nr_more_io = 0;

	nr_dirty = nr_io = nr_more_io = 0;

	spin_lock(&inode_wb_list_lock);

	list_for_each_entry(inode, &wb->b_dirty, i_wb_list)

		nr_dirty++;