Clear DISALLOW_ADD_USER in clearDeviceOwner for all cases

We are not moving the restriction from system to the DO in the end.
clearDeviceOwnerUserRestrictionLocked becomes the permanent solution
for DeviceOwner CTS. Looks like no one setting DISALLOW_ADD_USER
directly in UserManager except DO/PO, and so remove it when DO is
clear

Change-Id: I235bebebd02b5e0d9883eea6dd3a4e49b40fe043
Fix: 33476323
Test: runtest frameworks-services -c com.android.server.devicepolicy.DevicePolicyManagerTest