exfat: remove unneeded code from exfat_alloc_cluster()

In the removed code, num_clusters is 0, nothing is done in
exfat_chain_cont_cluster(), so it is unneeded, remove it.

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