Remove SetEnv.sh and install_fair_mpdroot.sh by the end of June 2022

EDIT: 2022-06-06 This will be done today, in case anyone wants to access these files, 
they are available in some older branch for example 
https://git.jinr.ru/nica/mpdroot/-/tree/ana_class

The website with howto is gone from the menu, but is still available online
https://mpdroot.jinr.ru/how-to-install-mpdroot-legacy-method/

• also remove links from the website and Readme.md

-- old build support ends in the end of June (we agreed upon that).
Additional new reason for it - FairSoft and FairRoot v 18.6.8, which will be default version in the June MPDroot release will use new ROOT v6.26 and new GEANT4 v11.
There is no way we can afford resources to support those for the old build.