caos.backends.pgsql: Do not crash on orphaned link indexes
caos.backends.pgsql: Do not crash on orphaned link indexes

A deficiency in pgsql backend delta engine can lead to orphaned indexes
to be left in the database after the corresponding link was dropped.  If
a link with the same name was reintroduced to the schema, pgsql delta
engine would crash with a KeyError.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb