Remove rlibmap that has been deprecated (and errored out) for 3 years.

core/utils/src/rlibmap.cxx

-#include <iostream>
-
-auto errMsg =
-"ERROR: The rlibmap utility is not available in ROOT6. In order to produce "
-"rootmap files you can use the genreflex utils (genreflex -h for more "
-"information) or the rootcling utility (rootcling -h for more information).";
-
-int main(){
-   std::cerr << errMsg << std::endl;
-   return 1;
-}