f2fs: remove dead code in update_meta_page

After commit a468f0ef

 ("f2fs: use crc and cp version to determine
roll-forward recovery"), last caller of update_meta_page passing @src
with NULL is gone, so remove related dead code there.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>