f2fs: remove wrong comment in f2fs_invalidate_page()

Since 8c242db9b8c0 ("f2fs: fix stale ATOMIC_WRITTEN_PAGE private pointer"),
we've started to not skip clear private flag for atomic_write page
truncation, so removing old wrong comment in f2fs_invalidate_page().

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