Update SCons setup to match podd setup and other cleanup

  Eliminate duplicated/redundant configuration code in SCons system
  Let SCons also clean ROOT 6's *_rdict.pcm class layout cache files
  Move helper python script to site_scons
  Remove EVIO from scons as podd build will take care of it.
  Remove define of HALLC_MODS in SConstruct
    (There is no longer any Hall C conditional code in Podd)