Revert "f2fs: add a new function get_ssr_cost"

This reverts commit b7b7c4cf.

se->ckpt_valid_blocks will never be smaller than se->valid_blocks, so just
remove get_ssr_cost.

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