cmGeneratorTarget: Don't assing a bool to a string

Source/cmGeneratorTarget.cxx

@@ -3945,7 +3945,6 @@ std::string cmGeneratorTarget::GetFortranModuleDirectory(
   std::string const& working_dir) const
 {
   if (!this->FortranModuleDirectoryCreated) {
-    this->FortranModuleDirectory = true;
     this->FortranModuleDirectory =
       this->CreateFortranModuleDirectory(working_dir);
   }