The get_writeback_state() has gone since 2006, kill related comments.

Link: https://lkml.kernel.org/r/20210508125026.56600-1-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>