staging: erofs: sunset erofs_workstation_cleanup_all

There is only one user calling erofs_workstation_cleanup_all,
and it is no likely that more users will use in that way
in the future.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>