Remove unused extension code

Assets/Scripts/Multipeer/SerializedARCollaborationDataExtensions.cs

-using NSData = Unity.iOS.Multipeer.NSData;
-
-namespace UnityEngine.XR.ARKit
-{
-    public static class SerializedARCollaborationDataExtensions
-    {
-        public static unsafe NSData ToNSData(this SerializedARCollaborationData serializedData)
-        {
-            return *(NSData*)&serializedData;
-        }
-
-        public static unsafe SerializedARCollaborationData ToSerializedCollaborationData(this NSData data)
-        {
-            return *(SerializedARCollaborationData*)&data;
-        }
-    }
-}

Assets/Scripts/Multipeer/SerializedARCollaborationDataExtensions.cs.meta

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