nm dns: Do not use interface DNS for static ip + static DNS

Previously, when desired state contains both static IP and static DNS,
nmstate will store the DNS at interface level, doing so require purging
DNS stored in undesired interfaces. This will cause network interruption
when nmstate reapply or reactivate this undesired interface.

The fix is remove the check on `static IP + static DNS` for whether
interface DNS is preferred/required.

Integration test case included.

Resolves: https://issues.redhat.com/browse/RHEL-125548

Signed-off-by: Gris Ge <fge@redhat.com>