Remove UnionsAvailable member from 2 classes

These classes were copied from cmDependsJavaParserHelper that really needs this
member.

Source/cmCommandArgumentParserHelper.h

@@ -81,7 +81,6 @@ private:
   cmStdString InputBuffer;
   std::vector<char> OutputBuffer;
   int CurrentLine;
-  int UnionsAvailable;
   int Verbose;
 
   void Print(const char* place, const char* str);

Source/cmExprParserHelper.h

@@ -55,7 +55,6 @@ private:
   cmStdString InputBuffer;
   std::vector<char> OutputBuffer;
   int CurrentLine;
-  int UnionsAvailable;
   int Verbose;
 
   void Print(const char* place, const char* str);