Revert "ipn/ipnlocal: be authoritative for the entire MagicDNS record tree."

Unfortunately this broke MagicDNS almost entirely.

Updates: tailscale/corp#1706

This reverts commit 1d7e7b49eb.

ipn/ipnlocal/local.go

@@ -1897,12 +1897,6 @@ func magicDNSRootDomains(nm *netmap.NetworkMap) []dnsname.FQDN {
 			// TODO: propagate error
 			return nil
 		}
-		if strings.HasSuffix(fqdn.WithoutTrailingDot(), ".beta.tailscale.net") {
-			// When using our standard MagicDNS suffix, make ourselves
-			// authoritative for all of tailscale.net, so that we
-			// become the resolver for shared nodes as well.
-			fqdn = dnsname.FQDN("tailscale.net.")
-		}
 		ret := []dnsname.FQDN{
 			fqdn,
 			dnsname.FQDN("0.e.1.a.c.5.1.1.a.7.d.f.ip6.arpa."),