exfat: remove useless check in exfat_move_file()

In exfat_move_file(), the identity of source and target directory has been
checked by the caller.
Also, it gets stream.start_clu from file dir-entry, which is an invalid
determination.

Signed-off-by: Tetsuhiro Kohada <kohada.t2@gmail.com>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>