cmCommand: remove member Helper

The Helper member is used in a single derived class only.
Remove it from the base class to the only place where it is used.

Source/cmExportCommand.h

@@ -41,6 +41,7 @@ public:
   std::string GetName() const CM_OVERRIDE { return "export"; }
 
 private:
+  cmCommandArgumentsHelper Helper;
   cmCommandArgumentGroup ArgumentGroup;
   cmCAStringVector Targets;
   cmCAEnabler Append;