Using cleanup

PerfectNumbers.Core/DivisorCycleCache.cs

@@ -1,10 +1,7 @@
-using System;
 using System.Buffers;
 using System.Collections.Concurrent;
-using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
-using System.Threading;
 using ILGPU;
 using ILGPU.Runtime;
 using PerfectNumbers.Core.Gpu;