erofs: remove obsoleted comments

Some comments haven't been useful anymore since the code updated.
Let's drop them instead.

Link: https://lore.kernel.org/r/20220506194612.117120-2-hsiangkao@linux.alibaba.com


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