WIP:Remove GAUDI_V20_COMPAT, HAVE_GAUDI_PLUGINSVC and ATLAS_GAUDI_V21 flags, fix for ATLINFR-1086
Fairly trivial, but GAUDI_V20_COMPAT is no longer used in master so it can be removed from the AthenaExternals build flags.
The HAVE_GAUDI_PLUGINSVC and ATLAS_GAUDI_V21 flags are still used but I am tackling this in https://gitlab.cern.ch/atlas/athena/merge_requests/8424
Cheers,
Stewart