Use IClassIDSvc header from GaudiKernel
Delete the "forwarding-header" AthenaKernel/IClassIDSvc.h and change all clients to use GaudiKernel/IClassIDSvc.h.
Delete the "forwarding-header" AthenaKernel/IClassIDSvc.h and change all clients to use GaudiKernel/IClassIDSvc.h.