Export: Clean up comment.

Introduced in commit a4263c9f (export(): Handle multiple dependent export
sets., 2013-10-10)

Source/cmExportBuildFileGenerator.cxx

@@ -229,10 +229,8 @@ cmExportBuildFileGenerator::HandleMissingTarget(
   // Append it with the export namespace.
   link_libs += this->Namespace;
   link_libs += dependee->GetExportName();
-//   if generate time {}
 }
 
-
 //----------------------------------------------------------------------------
 std::vector<std::string>
 cmExportBuildFileGenerator