AthDerivation: delete obsolete project

We do not use the AthDerivation project anymore in main. Derivations are produced with the full Athena project. If needed the project can be restored from git history.

Projects/AthDerivation/cmake/PostConfig.cmake.in

-# Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
-#
-# File taking care of pointing the downstream projects at the right
-# version of the externals.
-#
-
-# Set up a "quiet flag".
-set( _quietFlag )
-if( @CMAKE_PROJECT_NAME@_FIND_QUIETLY )
-   set( _quietFlag QUIET )
-endif()
-
-# CMake cache variable(s).
-set( CLHEP_ROOT "${CLHEP_LCGROOT}" CACHE PATH "Path to pick up CLHEP from" )
-
-# Find Gaudi.
-find_package( Gaudi @Gaudi_VERSION@ EXACT REQUIRED ${_quietFlag} )