erofs: clean up unnecessary code and comments

Some conditional macros and comments are useless.

Link: https://lore.kernel.org/r/20220927063607.54832-1-hsiangkao@linux.alibaba.com


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