exfat: Remove unused functions exfat_high_surrogate() and exfat_low_surrogate()

After applying previous two patches, these functions are not used anymore.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>