Currently, erofs_map_blocks() will be called only from
erofs_{bmap, read_raw_page} which are all for uncompressed files.
So, the compression branch in erofs_map_blocks() is pointless. Let's
remove it and use erofs_map_blocks_flatmode() directly. Also update
related comments.

Link: https://lore.kernel.org/r/20210325071008.573-1-zbestahu@gmail.comReviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NYue Hu <huyue2@yulong.com>
Signed-off-by: NGao Xiang <hsiangkao@redhat.com>