btrfs: tests: remove if duplicate in __check_free_space_extents()

num_extents is already checked in the next if condition and can
be safely removed.

Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>