C#: Replace Guards with the new implementation.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/Caching.qll

@@ -10,7 +10,6 @@ module Stages {
   cached
   module ControlFlowStage {
     private import semmle.code.csharp.controlflow.internal.Splitting
-    private import semmle.code.csharp.controlflow.Guards as Guards
 
     cached
     predicate forceCachingInSameStage() { any() }
@@ -21,8 +20,6 @@ module Stages {
       or
       exists(ControlFlow::Node n)
       or
-      Guards::Internal::isCustomNullCheck(_, _, _, _)
-      or
       forceCachingInSameStageRev()
     }
   }