cmMakefile: Remove unused SubDirectoryOrder member.

The last use was removed in v2.4.0~2054 (ENH: add support for out
of source source, 2005-03-14)

Source/cmMakefile.cxx

@@ -144,7 +144,6 @@ cmMakefile::cmMakefile(const cmMakefile& mf): Internal(new Internals)
   this->LocalGenerator = mf.LocalGenerator;
   this->FunctionBlockers = mf.FunctionBlockers;
   this->MacrosList = mf.MacrosList;
-  this->SubDirectoryOrder = mf.SubDirectoryOrder;
   this->Properties = mf.Properties;
   this->PreOrder = mf.PreOrder;
   this->WarnUnused = mf.WarnUnused;

Source/cmMakefile.h

@@ -1051,8 +1051,6 @@ private:
 
   std::vector<std::string> MacrosList;
 
-  std::map<std::string, bool> SubDirectoryOrder;
-
   mutable cmsys::RegularExpression cmDefineRegex;
   mutable cmsys::RegularExpression cmDefine01Regex;
   mutable cmsys::RegularExpression cmAtVarRegex;