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.go

@@ -291,16 +291,6 @@ type Prefs struct {
 	// non-nil.
 	RelayServerStaticEndpoints []netip.AddrPort `json:",omitempty"`
 
-	// AllowSingleHosts was a legacy field that was always true
-	// for the past 4.5 years. It controlled whether Tailscale
-	// peers got /32 or /128 routes for each other.
-	// As of 2024-05-17 we're starting to ignore it, but to let
-	// people still downgrade Tailscale versions and not break
-	// all peer-to-peer networking we still write it to disk (as JSON)
-	// so it can be loaded back by old versions.
-	// TODO(bradfitz): delete this in 2025 sometime. See #12058.
-	AllowSingleHosts marshalAsTrueInJSON
-
 	// The Persist field is named 'Config' in the file for backward
 	// compatibility with earlier versions.
 	// TODO(apenwarr): We should move this out of here, it's not a pref.
@@ -331,13 +321,6 @@ func (au1 AutoUpdatePrefs) Equals(au2 AutoUpdatePrefs) bool {
 		ok1 == ok2
 }
 
-type marshalAsTrueInJSON struct{}
-
-var trueJSON = []byte("true")
-
-func (marshalAsTrueInJSON) MarshalJSON() ([]byte, error) { return trueJSON, nil }
-func (*marshalAsTrueInJSON) UnmarshalJSON([]byte) error  { return nil }
-
 // AppConnectorPrefs are the app connector settings for the node agent.
 type AppConnectorPrefs struct {
 	// Advertise specifies whether the app connector subsystem is advertising