auth: Clean up redundant request.session.modified change.

This line was added in 94e099e,
presumably because of the
del request.session["multiuse_object_key"]
line that was just above it.
Looks like it should have been removed in
868a763, which eliminated that `del`
operation.

zerver/views/auth.py

@@ -250,7 +250,6 @@ def maybe_send_to_registration(
             )
 
         if multiuse_obj is not None:
-            request.session.modified = True
             streams_to_subscribe = list(multiuse_obj.streams.all())
             prereg_user.streams.set(streams_to_subscribe)
             prereg_user.invited_as = invited_as