Make sure Twitter user IDs are removed if screen name is removed
Make sure Twitter user IDs are removed if screen name is removed

This change also means that we won't get an error when trying to update
someone who had multiple Twitter user IDs in Identifier objects. (This
sometimes happens because the merge code currently creates multiple
identifiers with the same scheme.)

DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative