nss-resolve: drop the internal fallback to libnss_dns (#5945)

nss-resolve: drop the internal fallback to libnss_dns (#5945)

If we could not communicate with systemd-resolved, we would call into
libnss_dns. libnss_dns would return NOTFOUND for stuff like "localhost" and
other names resolved by nss-myhostname, which we would fall under the !UNAVAIL=
condition and cause resolution to fail. So the following recommended
configuration in nsswitch.conf would not work:

   hosts: resolve [!UNAVAIL=return] dns myhostname

Remove the internal fallback code completely so that the fallback logic
can be configured in nsswitch.conf.

Tested with
   hosts: resolve [!UNAVAIL=return] myhostname
and
   hosts: resolve [!UNAVAIL=return] dns myhostname

Fixes #5742.