exfat: drop ->i_size_ondisk

->i_size_ondisk is no longer used by exfat_write_begin() after
commit(11a347fb exfat: change to get file size from DataLength),
drop it.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>