flow: Cut workaround for facebook/flow#8833

This issue was fixed in Flow v0.173.0, after I reported it:
  facebook/flow#8833
  https://github.com/facebook/flow/blob/main/Changelog.md#01730

Now that we've upgraded past that (in #5700), we get to
drop the workaround.

Author: gnprice

src/message/messagesReducer.js

@@ -269,10 +269,7 @@ export default (
                 return oldMessage;
               }
 
-              // FlowIssue: https://github.com/facebook/flow/issues/8833
-              //   The cast `: 'stream'` is silly but harmless, and works
-              //   around a Flow issue which causes an error.
-              if (oldMessage.type !== ('stream': 'stream')) {
+              if (oldMessage.type !== 'stream') {
                 logging.warn('messagesReducer: got update_message with stream/topic move on PM');
                 return oldMessage;
               }