Convert: Remove UNCHANGED enum value

It is no longer used.

Source/cmCommonTargetGenerator.h

@@ -57,9 +57,9 @@ protected:
   // The windows module definition source file (.def), if any.
   cmSourceFile const* ModuleDefinitionFile;
 
-  std::string Convert(
-    std::string const& source, cmOutputConverter::RelativeRoot relative,
-    cmOutputConverter::OutputFormat output = cmOutputConverter::UNCHANGED);
+  std::string Convert(std::string const& source,
+                      cmOutputConverter::RelativeRoot relative,
+                      cmOutputConverter::OutputFormat output);
 
   void AppendFortranFormatFlags(std::string& flags,
                                 cmSourceFile const& source);

Source/cmOutputConverter.h

@@ -45,7 +45,6 @@ public:
   };
   enum OutputFormat
   {
-    UNCHANGED,
     MAKERULE,
     SHELL,
     WATCOMQUOTE,
@@ -54,7 +53,7 @@ public:
   std::string ConvertToOutputFormat(const std::string& source,
                                     OutputFormat output) const;
   std::string Convert(const std::string& remote, RelativeRoot local,
-                      OutputFormat output = UNCHANGED) const;
+                      OutputFormat output) const;
   std::string ConvertToRelativePath(const std::string& remote,
                                     RelativeRoot local) const;
   std::string ConvertDirectorySeparatorsForShell(