mocks: Remove __mocks__/redux-mock-store.js.

It looks like this was added with the idea that we'd use it in more
places than we actually have. It adds a slight bit of convenience;
with this mock, we can avoid having to write
`configureStore([thunk])` a bunch of times.

But that's a very small amount of boilerplate.

The mock doesn't seem to have any other purpose that might be part
of a unit-testing strategy.

Removing it makes it possible to use the libdef without contorting
it more drastically than we already have.

[cherry-picked from zulip#4171]

src/message/__tests__/fetchActions-test.js

@@ -1,9 +1,12 @@
-import mockStore from 'redux-mock-store'; // eslint-disable-line
+import configureStore from 'redux-mock-store';
+import thunk from 'redux-thunk';
 
 import { fetchMessages, fetchOlder, fetchNewer } from '../fetchActions';
 import { streamNarrow, HOME_NARROW, HOME_NARROW_STR } from '../../utils/narrow';
 import { navStateWithNarrow } from '../../utils/testHelpers';
 
+const mockStore = configureStore([thunk]);
+
 const narrow = streamNarrow('some stream');
 const streamNarrowStr = JSON.stringify(narrow);
 

src/message/__tests__/messageActions-test.js

@@ -1,9 +1,12 @@
-import mockStore from 'redux-mock-store'; // eslint-disable-line
+import configureStore from 'redux-mock-store';
+import thunk from 'redux-thunk';
 
 import { doNarrow } from '../messagesActions';
 import { streamNarrow, HOME_NARROW, privateNarrow } from '../../utils/narrow';
 import { navStateWithNarrow } from '../../utils/testHelpers';
 
+const mockStore = configureStore([thunk]);
+
 const streamNarrowObj = streamNarrow('some stream');
 const streamNarrowStr = JSON.stringify(streamNarrowObj);