Remove our own impl of ReadOnlyList and update usages

OpenRA.Mods.Cnc/SpriteLoaders/ShpRemasteredLoader.cs

@@ -10,6 +10,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Text.RegularExpressions;
@@ -111,7 +112,7 @@ namespace OpenRA.Mods.Cnc.SpriteLoaders
 				}
 			}
 
-			Frames = frames.AsReadOnly();
+			Frames = frames;
 		}
 	}
 }