Remove swimming

Swimming is obsolete. If it is used at all, it will be run at user level and not in production. See LHCBPS-1867. See also discussion in LHCb!2719 (merged) and in !763 (merged)

@pkoppenb @gligorov @eklund.

Addresses the clean-up campaign see https://gitlab.cern.ch/lhcb-dpa/project/-/issues/18.