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/prefs_test.go

@@ -302,7 +302,7 @@ func checkPrefs(t *testing.T, p Prefs) {
 	if p.Equals(p2) {
 		t.Fatalf("p == p2\n")
 	}
-	p2b, err = PrefsFromBytes(p2.ToBytes(), false)
+	p2b, err = PrefsFromBytes(p2.ToBytes())
 	if err != nil {
 		t.Fatalf("PrefsFromBytes(p2) failed\n")
 	}