cmMakefile: Remove unused Prefix member.

Unused since it was added in commit 1f42f521 (NEW: move from tools
and config to create CMake, 2000-08-29).

Source/cmMakefile.cxx

@@ -113,7 +113,6 @@ cmMakefile::cmMakefile(const cmMakefile& mf): Internal(new Internals)
   this->Internal->VarInitStack.push(mf.Internal->VarInitStack.top());
   this->Internal->VarUsageStack.push(mf.Internal->VarUsageStack.top());
 
-  this->Prefix = mf.Prefix;
   this->cmStartDirectory = mf.cmStartDirectory;
   this->StartOutputDirectory = mf.StartOutputDirectory;
   this->cmHomeDirectory = mf.cmHomeDirectory;

Source/cmMakefile.h

@@ -961,8 +961,6 @@ protected:
   // Check for a an unused variable
   void CheckForUnused(const char* reason, const std::string& name) const;
 
-  std::string Prefix;
-
   std::string cmStartDirectory;
   std::string StartOutputDirectory;
   std::string cmHomeDirectory;