f2fs: obsolete unused MAX_DISCARD_BLOCKS

After commit a7eeb823

 ("f2fs: use bitmap in discard_entry"),
MAX_DISCARD_BLOCKS became obsolete, remove it.

Signed-off-by: Chao Yu <chao.yu@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>