net/dns: remove PerDomain from Config.

It's currently unused, and no longer makes sense with the upcoming
DNS infrastructure. Keep it in tailcfg for now, since we need protocol
compat for a bit longer.

Signed-off-by: David Anderson <danderson@tailscale.com>

ipn/ipnlocal/local.go

@@ -1452,7 +1452,6 @@ func (b *LocalBackend) authReconfig() {
 		rcfg.DNS = dns.Config{
 			Nameservers: nm.DNS.Nameservers,
 			Domains:     nm.DNS.Domains,
-			PerDomain:   nm.DNS.PerDomain,
 			Proxied:     proxied,
 		}
 	}