staging: erofs: remove incomplete cleancache

cleancache was not fully implemented in EROFS.
In addition, it's tend to remove the whole cleancache in
related attempt [1].

[1] https://lore.kernel.org/linux-fsdevel/20190527103207.13287-3-jgross@suse.com/


Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190813023054.73126-2-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>