erofs: remove unused device mapping in meta routine

Currently metadata is always on bootstrap, and thus device mapping is
not needed so far.  Remove the redundant device mapping in the meta
routine.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230209063913.46341-2-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>