exfat: remove unused functions

exfat_count_ext_entries() is no longer called, remove it.
exfat_update_dir_chksum() is no longer called, remove it and
rename exfat_update_dir_chksum_with_entry_set() to it.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>