actually just remove undefined error as it is unused

Author: jonahm-LANL

singularity-opac/base/opac_error.hpp

@@ -27,9 +27,6 @@ namespace singularity {
   printf("%s", x);                                                             \
   std::exit(1)
 #endif
-#ifndef UNDEFINED_ERROR
-#define UNDEFINED_ERROR OPAC_ERROR("DEFINE ME\n")
-#endif // UNDEFINED_ERROR
 
 } // namespace singularity