You can demote the last domain controller in an Active Directory domain using the DCPromo command-line utility. Once it has been demoted you can safely remove the DNS Role from the server and reboot.
Demote Domain Controller
dcpromo /DemoteFSMO:Yes /IsLastDCInDomain:Yes /RemoveApplicationPartitions:Yes
/AdministratorPassword:Password!
Uninstall DNS Server Role
start /w ocsetup DNS-Server-Core-Role /uninstall
Reboot Server
shutdown -r -t 0
