Remove ancient special case code for adding oid columns

Remove ancient special case code for adding oid columns

The special handling of negative attribute numbers in
ATExecAddColumn() was introduced to support SET WITH OIDS (commit
6d1e361852).  But that feature doesn't exist anymore, so we can revert
to the previous, simpler version.  In passing, also remove an obsolete
comment about OID support.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da@eisentraut.org