C#: Make private.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll

@@ -2290,8 +2290,7 @@ module Internal {
      *
      * This predicate relies on the control flow graph.
      */
-    cached
-    predicate impliesStep(Guard g1, AbstractValue v1, Guard g2, AbstractValue v2) {
+    private predicate impliesStep(Guard g1, AbstractValue v1, Guard g2, AbstractValue v2) {
       preImpliesStep(g1, v1, g2, v2)
       or
       forex(ControlFlow::Node cfn1 | cfn1 = g1.getAControlFlowNode() |