was_defendant_served method should only check "Service" case events

[ThomasThoren]

The was_defendant_served method checks for "Served" dates across all case event types. It should only check for the "Service" case event.

For example, case J2-CV-20-000021 has two events with "Served" dates: "Writ" and "Service". The was_defendant_served method would return the "Served" date of the "Writ" event instead of the "Service" event.

Also, the was_defendant_served method is only used in the test suite. Is the method needed any longer?

[mscarey]

I think the was_defendant_served method is still needed because service of process was issue #6. It's possible the method only ever needed to return a boolean rather than a date.