core: don't restrict DNS interface when performing connectivity check

Currently, when performing DNS resolution with systemd-resolved, NetworkManager tells systemd-resolved to consider only DNS configuration for the network interface that the connectivity check request will be routed through. But this is not correct because DNS and routing are configured entirely separately. For example, say we have a VPN that receives all DNS but only a subset of routing. NetworkManager will configure systemd-resolved with no DNS servers on any interface except for the VPN interface, but will still route traffic through other interfaces. This is entirely legitimate and works fine in practice, except for the connectivity check.

To fix this, we just drop the restriction and allow systemd-resolved to consider its full configuration, which is what gets used normally anyway. This allows our connectivity check to match the real configuration instead of failing spuriously.

Fixes #1107 (closed)