removed unused using

Author: StephenHodgson

Assets/MixedRealityToolkit/_Core/Interfaces/InputSystem/IMixedRealityFocusProvider.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
-using System.Collections.Generic;
 using Microsoft.MixedReality.Toolkit.Core.Definitions.Physics;
 using Microsoft.MixedReality.Toolkit.Core.EventDatum.Input;
 using Microsoft.MixedReality.Toolkit.Core.Interfaces.InputSystem.Handlers;