Replace custom mono defines with toolchain-provided defines.

OpenRA.Game/Support/AssemblyLoader.cs

@@ -11,7 +11,7 @@
 
 // Not used/usable on Mono. Only used for Dotnet Core.
 // Based on https://github.com/natemcmaster/DotNetCorePlugins and used under the terms of the Apache 2.0 license
-#if !MONO
+#if NET5_0_OR_GREATER
 using System;
 using System.Collections.Generic;
 using System.IO;