Doc: remove obsolete description of oid column as being "hidden".

Looks like one copy of this text didn't get updated.

Justin Pryzby

Discussion: https://postgr.es/m/20190701155932.GA22866@telsasoft.com

Author: tglsfdc

doc/src/sgml/catalogs.sgml

@@ -615,7 +615,7 @@
       <entry><structfield>oid</structfield></entry>
       <entry><type>oid</type></entry>
       <entry></entry>
-      <entry>Row identifier (hidden attribute; must be explicitly selected)</entry>
+      <entry>Row identifier</entry>
      </row>
 
      <row>