erofs: remove unnecessary output in erofs_show_options()

erofs: remove unnecessary output in erofs_show_options()

We have already handled cache_strategy option carefully,
so incorrect setting could not pass option parsing.
Meanwhile, print 'cache_strategy=(unknown)' can cause
failure on remount.

Link: https://lore.kernel.org/r/20191119115049.3401-1-cgxu519@mykernel.net
Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>