C#: Remove `DefUse.qll`

csharp/ql/src/semmle/code/csharp/dataflow/DefUse.qll

@@ -1,8 +0,0 @@
-/**
- * Provides classes and predicates for working with variable definitions and uses.
- */
-
-// This file is kept for backwards compatibility
-
-import csharp
-import SSA

csharp/ql/test/library-tests/dataflow/callablereturnsarg/Common.qll

@@ -1,5 +1,4 @@
 import csharp
-import semmle.code.csharp.dataflow.DefUse
 import semmle.code.csharp.controlflow.Guards
 
 class Configuration extends DataFlow::Configuration {