You can add subnets in the QoS manager, so that you can apply policies. When you right-click a subnet in QoS manager, and press Delete, only the associated policies are deleted.
Subnets are used by other components and the Qos has no way of knowing where.
To delete a subnet:
1. Open the Active Directory Sites and Services snap-in, from the Administrative Tools menu.
2. Expand Sites.
3. Expand Subnets.
4. Right-click the subnet you wish to delete and press Delete.
5. Respond Yes to the verification prompt.