Remove unused cmSourceGroup method

The non-const GetSourceFiles method is not needed.

Source/cmSourceGroup.cxx

@@ -119,12 +119,6 @@ const std::vector<const cmSourceFile*>& cmSourceGroup::GetSourceFiles() const
   return this->SourceFiles;
 }
 
-//----------------------------------------------------------------------------
-std::vector<const cmSourceFile*>& cmSourceGroup::GetSourceFiles()
-{
-  return this->SourceFiles;
-}
-
 //----------------------------------------------------------------------------
 void cmSourceGroup::AddChild(cmSourceGroup child)
 {

Source/cmSourceGroup.h

@@ -100,7 +100,6 @@ public:
    * source group.
    */
   const std::vector<const cmSourceFile*>& GetSourceFiles() const;
-  std::vector<const cmSourceFile*>& GetSourceFiles();
   
   std::vector<cmSourceGroup> const& GetGroupChildren() const;
 private: