C#: Address more review comments.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/dataflow/internal/TaintTrackingPrivate.qll

@@ -7,7 +7,6 @@ private import semmle.code.csharp.dataflow.internal.DataFlowPrivate
 private import semmle.code.csharp.dataflow.internal.ControlFlowReachability
 private import semmle.code.csharp.dispatch.Dispatch
 private import semmle.code.csharp.commons.ComparisonTest
-private import semmle.code.csharp.commons.Collections as Collections
 // import `TaintedMember` definitions from other files to avoid potential reevaluation
 private import semmle.code.csharp.frameworks.JsonNET
 private import semmle.code.csharp.frameworks.WCF