Simplify User.display_name a bit.

discord/user.py

@@ -133,6 +133,5 @@ class User:
         if they have a server specific nickname then that
         is returned instead.
         """
-        nickname = getattr(self, 'nick', None)
-        return nickname if nickname is not None else self.name
+        return getattr(self, 'nick', self.name)