In the most brief terms..... "
Leave it alone....it belongs to the OS, (8.1)"
What belongs to the OS? The 21.85 GB of free space?
The OS doesn't / can't use the " Free Space " as it doesn't have a partition and isn't formated....
TechoMage is correct.
Not really sure how OP installed Windows in an extended partiton with
C: as a logical drive so if you delete the partition, you
also delete the logical drive C: within the extended partition which contains Windows OS.