ipn: remove enforceDefaults option from PrefsFromBytes

The Mac client was using it, but it had the effect of the `RouteAll`
("Use Tailscale subnets") pref always being enabled at startup,
regardless of the persisted value.

enforceDefaults was added to handle cases from ~2 years ago where
we ended up with persisted `"RouteAll": false` values in the keychain,
but that should no longer be a concern. New users will get the default
of it being enabled via `NewPrefs`.

There will be a corresponding Mac client change to stop passing in
enforceDefaults.

For #3962

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

ipn/ipnlocal/local.go

@@ -1617,7 +1617,7 @@ func (b *LocalBackend) loadStateLocked(key ipn.StateKey, prefs *ipn.Prefs) (err
 	case err != nil:
 		return fmt.Errorf("backend prefs: store.ReadState(%q): %v", key, err)
 	}
-	b.prefs, err = ipn.PrefsFromBytes(bs, false)
+	b.prefs, err = ipn.PrefsFromBytes(bs)
 	if err != nil {
 		b.logf("using backend prefs for %q", key)
 		return fmt.Errorf("PrefsFromBytes: %v", err)