cmListFile: Remove unused member.

It is written, but not read since commit v2.4.0~575 (ENH: Since list file
cache does not make much sense any more ..., 2006-02-07).

Source/cmListFileCache.cxx

@@ -146,18 +146,12 @@ bool cmListFile::ParseFile(const char* filename,
     }
 
   bool parseError = false;
-  this->ModifiedTime = cmSystemTools::ModifiedTime(filename);
 
   {
   cmListFileParser parser(this, mf, filename);
   parseError = !parser.ParseFile();
   }
 
-  if(parseError)
-    {
-    this->ModifiedTime = 0;
-    }
-
   // do we need a cmake_policy(VERSION call?
   if(topLevel)
   {

Source/cmListFileCache.h

@@ -85,15 +85,10 @@ class cmListFileBacktrace: public std::vector<cmListFileContext>
 
 struct cmListFile
 {
-  cmListFile()
-    :ModifiedTime(0)
-    {
-    }
   bool ParseFile(const char* path,
                  bool topLevel,
                  cmMakefile *mf);
 
-  long int ModifiedTime;
   std::vector<cmListFileFunction> Functions;
 };