caos.backends.pgsql.delta: Fix link rebase when a link obtains a table
caos.backends.pgsql.delta: Fix link rebase when a link obtains a table

The previous (ca8ca70703f) fix in this area was overzealous, as it is
not required for a link table to exist before the rebase.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb