ipn: remove the last traces of Prefs.AllowSingleHosts

We stopped reading this field nearly two years ago, with a TODO comment
to remove it sometime in 2025.

It is now 2026.

Updates #12058

Change-Id: I8ddf1c2e4c3c428e8d45a6491d3899368ec52c30
Signed-off-by: Alex Chan <alexc@tailscale.com>

ipn/prefs_test.go

@@ -70,7 +70,6 @@ func TestPrefsEqual(t *testing.T) {
 		"DriveShares",
 		"RelayServerPort",
 		"RelayServerStaticEndpoints",
-		"AllowSingleHosts",
 		"Persist",
 	}
 	if have := fieldsOf(reflect.TypeFor[Prefs]()); !reflect.DeepEqual(have, prefsHandles) {
@@ -714,7 +713,7 @@ func TestMaskedPrefsFields(t *testing.T) {
 	have := map[string]bool{}
 	for _, f := range fieldsOf(reflect.TypeFor[Prefs]()) {
 		switch f {
-		case "Persist", "AllowSingleHosts":
+		case "Persist":
 			// These can't be edited.
 			continue
 		}
@@ -1222,27 +1221,6 @@ func TestNotifyPrefsJSONRoundtrip(t *testing.T) {
 	}
 }
 
-// Verify that our Prefs type writes out an AllowSingleHosts field so we can
-// downgrade to older versions that require it.
-func TestPrefsDowngrade(t *testing.T) {
-	var p Prefs
-	j, err := json.Marshal(p)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	type oldPrefs struct {
-		AllowSingleHosts bool
-	}
-	var op oldPrefs
-	if err := json.Unmarshal(j, &op); err != nil {
-		t.Fatal(err)
-	}
-	if !op.AllowSingleHosts {
-		t.Fatal("AllowSingleHosts should be true")
-	}
-}
-
 func TestParseAutoExitNodeString(t *testing.T) {
 	tests := []struct {
 		name       string