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/cmCommand.h

@@ -7,7 +7,6 @@
 #include <string>
 #include <vector>
 
-#include "cmCommandArgumentsHelper.h"
 #include "cmPolicies.h"
 
 class cmExecutionStatus;
@@ -110,7 +109,6 @@ public:
 
 protected:
   cmMakefile* Makefile;
-  cmCommandArgumentsHelper Helper;
 
 private:
   std::string Error;

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;