Revert "resolved: filter out our own stub resolvers when parsing servers"

Revert "resolved: filter out our own stub resolvers when parsing servers"

This reverts commit 0ad4efb14beea9148838a0d974821e3b98cafc47.

See
https://github.com/systemd/systemd/pull/20559#issuecomment-1028011030
for reasoning. Quoting:

> I think it should be OK to advertise extra stub listeners to local
> clients, but you prohibit this now. i.e. there are two different
> concepts here, and we shouldn't mix them up:
>
> 1. tracking configured dns servers and advertise them to local programs
> 2. actually using them ourselves
>
> I am pretty sure that our own stubs are OK for 1 but not OK for 2,
> hence we should filter at the time of use not at the time of parse.