f2fs: swap: remove dead codes

After commit af4b6b8e ("f2fs: introduce check_swap_activate_fast()"),
we will never run into original logic of check_swap_activate() before
f2fs supports non 4k-sized page, so let's delete those dead codes.

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