wip narrow: getNarrowFromMessage never a stream narrow

There's no reason we would sometimes want a stream narrow here.
This is just how it was when this function was first written, for
narrowing to a message's conversation, in 34ab1f9 (zulip#333) in 2017.

This case also should be impossible anyway, because message topics
should always be nonempty -- we convert empty to "(no topic)".

src/utils/__tests__/narrow-test.js

@@ -365,17 +365,6 @@ describe('getNarrowFromMessage', () => {
     expect(actualNarrow).toEqual(expectedNarrow);
   });
 
-  test('if recipient of a message is string, returns a stream narrow', () => {
-    const message = {
-      display_recipient: 'stream',
-    };
-    const expectedNarrow = streamNarrow('stream');
-
-    const actualNarrow = getNarrowFromMessage(message);
-
-    expect(actualNarrow).toEqual(expectedNarrow);
-  });
-
   test('if recipient is a string and there is a subject returns a topic narrow', () => {
     const message = {
       display_recipient: 'stream',

src/utils/narrow.js

@@ -271,9 +271,5 @@ export const getNarrowFromMessage = (message: Message | Outbox, ownEmail: string
     return groupNarrow(recipient.map(x => x.email));
   }
 
-  if (message.subject && message.subject.length) {
-    return topicNarrow(message.display_recipient, message.subject);
-  }
-
-  return streamNarrow(message.display_recipient);
+  return topicNarrow(message.display_recipient, message.subject);
 };