[System][Test] Only Xamarin.iOS has a synchronization context by default.

This shows up when Xamarin.Mac tests start defining XAMMAC [1]

[1]: dotnet/macios#3034

Author: rolfbjarne

mcs/class/System/Test/System.ComponentModel/AsyncOperationManagerTest.cs

@@ -26,7 +26,7 @@ public void SyncContext ()
 			SynchronizationContext sc1 = new SynchronizationContext ();
 			SynchronizationContext sc2 = new SynchronizationContext ();
 
-#if MONOTOUCH || XAMMAC
+#if MONOTOUCH
 			Assert.IsNotNull (SynchronizationContext.Current, "A1");
 #else
 			Assert.IsNull (SynchronizationContext.Current, "A1");