Unify MediaGlobal/Scripting.Media PlayFMV* methods

- Improve consistency
- Remove dead code
- Removed AsyncAction!
- Delegate.BeginInvoke is unsupported since .NET Core and throws an "Operation is not supported on this platform." exception.

OpenRA.Game/Effects/AsyncAction.cs

@@ -1,39 +0,0 @@
-#region Copyright & License Information
-/*
- * Copyright (c) The OpenRA Developers and Contributors
- * This file is part of OpenRA, which is free software. It is made
- * available to you under the terms of the GNU General Public License
- * as published by the Free Software Foundation, either version 3 of
- * the License, or (at your option) any later version. For more
- * information, see COPYING.
- */
-#endregion
-
-using System;
-using System.Collections.Generic;
-using OpenRA.Graphics;
-
-namespace OpenRA.Effects
-{
-	public class AsyncAction : IEffect
-	{
-		readonly Action a;
-		readonly IAsyncResult ar;
-
-		public AsyncAction(IAsyncResult ar, Action a)
-		{
-			this.a = a;
-			this.ar = ar;
-		}
-
-		public void Tick(World world)
-		{
-			if (ar.IsCompleted)
-			{
-				world.AddFrameEndTask(w => { w.Remove(this); a(); });
-			}
-		}
-
-		public IEnumerable<IRenderable> Render(WorldRenderer r) { yield break; }
-	}
-}