f2fs: remove unused check on version_bitmap

A NULL will not be return by __bitmap_ptr here.
Remove the unused check.

Signed-off-by: Wang Xiaojun <wangxiaojun11@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>