TS-1006: Remove unused InkFreeList->offset

I have simplified ink_freelist_init API by removing offset parameter,
but I forgot to remove unused InkFreeList->offset when rebasing code.

So now, the code using an uninitialized value, it would be a bug. Let's
clean it completely.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
Signed-off-by: Zhao Yongming <ming.zym@gmail.com>