presence model tests: Remove one that expects fallback we don't need

We removed the explicit fallback code this was testing in b945442.
As it turns out, the way we use Immutable.Map (new in that commit's
same PR) effectively handles the ancient-server behavior as well as
the explicit fallback was, and so this test still passes. But that's
incidental and not something we need to guarantee.

src/presence/__tests__/presenceModel-test.js

@@ -224,28 +224,6 @@ describe('presenceReducer', () => {
       const expectedState = makePresenceState([[user, userPresence]]);
       expect(presenceReducer(prevState, action)).toEqual(expectedState);
     });
-
-    // TODO(#5102): Delete; see comment on implementation.
-    test('when no `presence` data is given reset state', () => {
-      const user = eg.otherUser;
-      const userPresence = {
-        aggregated: { client: 'website', status: 'active', timestamp: 123 },
-        website: { client: 'website', status: 'active', timestamp: 123 },
-      };
-      const prevState = makePresenceState([[user, userPresence]]);
-      const action = eg.mkActionRegisterComplete({
-        // Hmm, we should need a Flow suppression here. This property is
-        // marked required in InitialData, and this explicit undefined is
-        // meant to defy that; see TODO(#5102) above.
-        // mkActionRegisterComplete is designed to accept input with this or
-        // any property *omitted*… and I think, as a side effect of handling
-        // that, Flow mistakenly accepts an explicit undefined here, so it
-        // doesn't catch the resulting malformed InitialData.
-        presences: undefined,
-      });
-      const expectedState = makePresenceState([]);
-      expect(presenceReducer(prevState, action)).toEqual(expectedState);
-    });
   });
 
   describe('PRESENCE_RESPONSE', () => {