fixup! Improve tracking toolset of improperly released unmanaged handles

Author: nulltoken

LibGit2Sharp/Core/Handles/SafeHandleBase.cs

@@ -19,7 +19,6 @@
 //#define LEAKS_TRACKING
 
 using System;
-using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;