Remove obsolete SwimmingEvent

Marco Cattaneorequested to merge
removeSwimmingEvent into master

Swimming will not be used in Run 3, see LHCBPS-1867

Should not be merged before Phys!763 (merged)

Depends on Phys!767 (merged)

@pkoppenb @gligorov FYI

Edited by Milosz Zdybal

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