Remove our own ReadOnlyDictionary and update usages

OpenRA.Mods.Cnc/FileSystem/MixFile.cs

@@ -11,6 +11,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.IO;
 using System.Linq;
 using OpenRA.FileSystem;

OpenRA.Mods.Cnc/Traits/PaletteEffects/ChronoshiftPaletteEffect.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System.Collections.Generic;
 using OpenRA.Graphics;
 using OpenRA.Primitives;
 using OpenRA.Traits;

OpenRA.Mods.Cnc/Traits/TDGunboat.cs

@@ -10,6 +10,7 @@
 #endregion
 
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Linq;
 using OpenRA.Activities;
 using OpenRA.Mods.Common;
@@ -42,7 +43,7 @@ namespace OpenRA.Mods.Cnc.Traits
 		public IReadOnlyDictionary<CPos, SubCell> OccupiedCells(ActorInfo info, CPos location, SubCell subCell = SubCell.Any)
 		{
 			var occupied = new Dictionary<CPos, SubCell>() { { location, SubCell.FullCell } };
-			return new ReadOnlyDictionary<CPos, SubCell>(occupied);
+			return occupied;
 		}
 
 		bool IOccupySpaceInfo.SharesCell => false;