net/dns/resolver: remove maxDoHInFlight

It was originally added to control memory use on iOS (#2490), but then
was relaxed conditionally when running on iOS 15 (#3098). Now that we
require iOS 15, there's no need for the limit at all, so simplify back
to the original state.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/dns/resolver/doh_test.go

@@ -45,9 +45,7 @@ func TestDoH(t *testing.T) {
 		t.Fatal("no known DoH")
 	}
 
-	f := &forwarder{
-		dohSem: make(chan struct{}, 10),
-	}
+	f := &forwarder{}
 
 	for _, urlBase := range prefixes {
 		t.Run(urlBase, func(t *testing.T) {