writeback: remove inode_to_wb_is_valid()

inode_to_wb_is_valid() is no longer used since commit fe55d563
("remove inode_congested()"), remove it.

Link: https://lkml.kernel.org/r/20220714084147.140324-1-xiujianfeng@huawei.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>