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/forwarder_test.go

@@ -12,7 +12,6 @@ import (
 	"time"
 
 	dns "golang.org/x/net/dns/dnsmessage"
-	"tailscale.com/hostinfo"
 	"tailscale.com/types/dnstype"
 )
 
@@ -143,33 +142,6 @@ func TestGetRCode(t *testing.T) {
 	}
 }
 
-func TestMaxDoHInFlight(t *testing.T) {
-	tests := []struct {
-		goos string
-		ver  string
-		want int
-	}{
-		{"ios", "", 10},
-		{"ios", "1532", 10},
-		{"ios", "9.3.2", 10},
-		{"ios", "14.3.2", 10},
-		{"ios", "15.3.2", 1000},
-		{"ios", "20.3.2", 1000},
-		{"android", "", 1000},
-		{"darwin", "", 1000},
-		{"linux", "", 1000},
-	}
-	for _, tc := range tests {
-		t.Run(fmt.Sprintf("%s-%s", tc.goos, tc.ver), func(t *testing.T) {
-			hostinfo.SetOSVersion(tc.ver)
-			got := maxDoHInFlight(tc.goos)
-			if got != tc.want {
-				t.Errorf("got %d; want %d", got, tc.want)
-			}
-		})
-	}
-}
-
 var testDNS = flag.Bool("test-dns", false, "run tests that require a working DNS server")
 
 func TestGetKnownDoHClientForProvider(t *testing.T) {