Remove a duplicate output display name
Remove a duplicate output display name

`dmg_by_event` was repeated in the dict of display names; remove the second appearance.

Former-commit-id: c89b258babfc2aa3be7b7262d2bfa0669e8f567e
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine