Remove support for array-valued store module state

All modules now use objects for their initial state, and this is a
recommended practice for extensibility, so remove support for store
modules using an array as their initial state.

src/sidebar/store/create-store.js

@@ -95,13 +95,6 @@ function createReducer(reducers) {
       return state;
     }
     const stateChanges = reducer(state, action);
-
-    // Some modules return an array rather than an object. They need to be
-    // handled differently so we don't convert them to an object.
-    if (Array.isArray(stateChanges)) {
-      return stateChanges;
-    }
-
     return {
       ...state,
       ...stateChanges,
@@ -286,7 +279,9 @@ export function makeAction(reducers, type, payload) {
  * @template Actions
  * @template {SelectorMap<State>} Selectors
  * @template [RootSelectors={}]
- * @param {State | ((...args: any[]) => State)} initialState
+ * @param {State | ((...args: any[]) => State)} initialState - Object containing
+ *   the initial state for the module, or a function which returns such an
+ *   object. The arguments come from the {@link createStore} call.
  * @param {object} config
  *   @param {string} config.namespace -
  *     The key under which this module's state will live in the store's root state

src/sidebar/store/test/create-store-test.js

@@ -65,29 +65,6 @@ const counterModules = [
   }),
 ];
 
-// Store module whose state is an array.
-const tagsModule = createStoreModule([], {
-  namespace: 'tags',
-
-  reducers: {
-    ADD_TAG(state, action) {
-      return [...state, action.tag];
-    },
-  },
-
-  actionCreators: {
-    addTag(tag) {
-      return { type: 'ADD_TAG', tag };
-    },
-  },
-
-  selectors: {
-    getTags(state) {
-      return state;
-    },
-  },
-});
-
 // Store module with reducers that update only a subset of the state.
 const groupsModule = createStoreModule(
   {
@@ -245,14 +222,6 @@ describe('createStore', () => {
     assert.equal(store.getState().test.value, 42);
   });
 
-  it('supports modules whose state is an array', () => {
-    const store = createStore([tagsModule]);
-    assert.deepEqual(store.getTags(), []);
-    store.addTag('tag-1');
-    store.addTag('tag-2');
-    assert.deepEqual(store.getTags(), ['tag-1', 'tag-2']);
-  });
-
   it('combines state updates from reducers with initial module state', () => {
     const store = createStore([groupsModule]);