narrow: Make getNarrowForReply stop returning stream narrows.

There's no reason we would sometimes want a stream narrow here. This
is just how it was when this function was first written (as
`getNarrowFromMessage`), 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)".

Co-authored-by: Chris Bobbe <cbobbe@zulip.com>

src/utils/__tests__/narrow-test.js

@@ -322,15 +322,6 @@ describe('getNarrowForReply', () => {
     expect(actualNarrow).toEqual(expectedNarrow);
   });
 
-  test('if recipient of a message is string, returns a stream narrow', () => {
-    const message = eg.streamMessage({ subject: '' });
-    const expectedNarrow = streamNarrow(eg.stream.name);
-
-    const actualNarrow = getNarrowForReply(message, ownEmail);
-
-    expect(actualNarrow).toEqual(expectedNarrow);
-  });
-
   test('if recipient is a string and there is a subject returns a topic narrow', () => {
     const message = eg.streamMessage();
     const expectedNarrow = topicNarrow(eg.stream.name, message.subject);

src/utils/narrow.js

@@ -324,9 +324,5 @@ export const getNarrowForReply = (message: Message | Outbox, ownEmail: string) =
     }
   }
 
-  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);
 };