The added/removed signals for the urlmark managers are no longer used as
the completion models are generated on-the-fly. The changed signal is
still needed so the save-manager knows when to trigger a write to disk.
Also removes session_manager.update_completion, which is no longer
needed for the same reason as above.
keyconf.changed cannot be removed, as it is still wired up to
basekeyparser.
Resolves#2874.
8e34b54cd7