erofs: get rid of debug_one_dentry()

Since erofsdump is available, no need to keep this debugging
functionality at all.

Also drop a useless comment since it's the VFS behavior.

Link: https://lore.kernel.org/r/20230114125746.399253-1-xiang@kernel.org


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>