Revert "f2fs: use flush command instead of FUA for zoned device"

This reverts commit c550e25b.

Commit c550e25b ("f2fs: use flush
command instead of FUA for zoned device") used additional flush
command to keep write order.

Since Commit dd291d77 ("block:
Introduce zone write plugging") has enabled the block layer to
handle this order issue, there is no need to use flush command.

Signed-off-by: Wenjie Cheng <cwjhust@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>