mute: Remove ancient-server support from the reasons for a fallback

src/mute/__tests__/muteModel-test.js

@@ -95,16 +95,6 @@ describe('reducer', () => {
       expect(newState).toBeTruthy();
       expect(newState && getMute(newState)).toEqual(makeMuteState([[eg.stream, 'topic']]));
     });
-
-    // TODO(#5102): Delete; see comment on implementation.
-    test('in ancient no-muted-topics format', () => {
-      const state = makeMuteState([[eg.stream, 'topic']]);
-      const action = eg.mkActionRegisterComplete({
-        muted_topics: undefined,
-        user_topics: undefined,
-      });
-      expect(reducer(state, action, eg.plusReduxState)).toEqual(initialState);
-    });
   });
 
   describe('RESET_ACCOUNT_DATA', () => {

src/mute/muteModel.js

@@ -111,12 +111,10 @@ export const reducer = (
       // TODO(server-6.0): Stop caring about that, when we cut muted_topics.
       return convertLegacy(
         action.data.muted_topics
-          // TODO(#5102): Delete fallback once we enforce any threshold for
-          //   ancient servers we refuse to connect to. It was added in
-          //   #2878 (2018-11-16), but it wasn't clear even then, it seems,
-          //   whether any servers actually omit the data. The API doc
-          //   doesn't mention any servers that omit it, and our Flow types
-          //   mark it required.
+          // Unnecessary fallback just to satisfy Flow: the old
+          // `muted_topics` is absent only when the new `user_topics` is
+          // present (ignoring ancient unsupported servers), but Flow
+          // doesn't track that.
           ?? [],
         getStreamsByName(globalState),
       );