bdi: remove enum wb_congested_state

enum wb_congested_state and the member 'congested' in bdi_writeback are
useless since commit a88f2096 ("remove congestion tracking
framework"), so remove it.

Link: https://lkml.kernel.org/r/20220719083349.87547-1-xiujianfeng@huawei.com


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: NeilBrown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>