In MediaWiki 1.21 the ss_admins field was removed as part of a schema change. Both the standard layout was updated and schema changes for existing installs, except for Postgres.
The new installation schema for Postgres still has the field, and existing installations didn't get a patch either.
rMWbd602d20833f: Drop unused database field ss_admins (https://gerrit.wikimedia.org/r/24675).