md/raid5-cache: removes unnecessary write-through mode judgments

The write-through mode has been returned in front of the function,
do not need to do it again.

Signed-off-by: JackieLiu <liuyun01@kylinos.cn>
Reviewed-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>