PanTau code cleanup

Hello,

This MR is cleaning up the PanTau code, which is no longer maintained but still used in the tau reconstruction. This is just to improve code readability for a possible rework or simplification of the code, which might be attempted before the reprocessing (but the code is so complex that it might not happen...). I dropped a huge number of ATH_MSG_DEBUG and lines that were commented out. I also dropped a few calculations that are made internally but used nowhere. This MR does not change the reconstruction output.

Cheers, Bertrand