Drop arn_value and arn_source from the authority.record_entry table

Drop arn_value and arn_source from the authority.record_entry table

Now that we have the unique index by thesaurus on actual headings, arn_value
and arn_source are a dead weight that we can gleefully toss overboard.

Tested with authority record import via Vandelay and authority record
creation during bibliographic editing; confirmed that validating authorities
still works, no apparent visible difference to the world.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@17706 dcc99617-32d9-48b4-a31d-7c20da2025e4