folder_branch_ops: remove obsolete XXX comment
folder_branch_ops: remove obsolete XXX comment

Issue: KBFS-2071

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client