exfat: remove count used cluster from exfat_statfs()

The callback function statfs() is called only after the file
system is mounted. During the process of mounting the exFAT
file system, the number of used clusters has been counted, so
the condition "sbi->used_clusters == EXFAT_CLUSTERS_UNTRACKED"
is always false and should be deleted.

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