outbox [nfc]: Clean up an old workaround for lacking sender_id

This property is required (since 60847c9 / #4667, earlier this
year), so we can rely on it to simplify away this workaround.

This workaround was in fact the original prompt that caused me to
notice we didn't have sender_id (or stream_id) on Outbox, and file
  #3968 (comment)

Author: gnprice

src/webview/html/message.js

@@ -231,8 +231,7 @@ $!${divOpenHtml}
 `;
   }
 
-  const { sender_full_name } = message;
-  const sender_id = message.isOutbox ? backgroundData.ownUser.user_id : message.sender_id;
+  const { sender_full_name, sender_id } = message;
   const avatarUrl = message.avatar_url
     .get(
       // 48 logical pixels; see `.avatar` and `.avatar img` in