thisroot.sh no longer remove previous version from the paths

[pcanal]

As reported at https://root-forum.cern.ch/t/the-thisroot-sh-misbahes-in-root-6-34/64513, since v6.34.00, thisroot.sh no longer remove the locations of a previously setup version of ROOT from any of the paths.

This was introduce in #16027 where the code pattern should actually be:

      if [ -n "${LD_LIBRARY_PATH-}" ]; then
         drop_from_path "$LD_LIBRARY_PATH" "${old_rootsys}/lib"
         drop_from_path "$newpath" "${old_rootsys}/lib/root"
         LD_LIBRARY_PATH=$newpath
      fi

[guitargeek]

Thanks for the report! I just wanted to know that one day (ROOT 7?) I hope to get rid of the thisroot scripts. They are quite fragile anyway, because they assume that nobody is messing with the CMAKE_<bla>_INSTALL_PATHs, which packagers do more and more.

Regressions should be fixed of course, but I wanted to advocate to not spend too much time on fixing new corner cases for these scripts later.