Revert "Permit 'select distinct on' raw SQL (See #11054, #9496)"

This reverts commit e71f936b6d24710a7bf4c03e19340c06c25cacf0.

This workaround was intended only for allowing use of SQL's
"select distinct on (...)". Since another solution was found
for the same problem (namely dropping all the intermediate
IDs from the select list), this need no longer be permitted.