udf: remove unused variables group_start and nr_groups

Variables group_start and nr_groups are being assigned but are never used
hence they are redundant and can be removed.

Cleans up clang warning:
variable 'group_start' set but not used [-Wunused-but-set-variable]
variable 'nr_groups' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jan Kara <jack@suse.cz>