Fix parsing of qualified relation names in RETURNING.

Fix parsing of qualified relation names in RETURNING.

Given a qualified refname, refnameNamespaceItem() will search for a
matching namespace item by relation OID, rather than by name. Commit
80feb727c8 broke this by adding additional namespace items for OLD and
NEW in the RETURNING list, which have the same relation OID, causing
ambiguity. Fix this by ignoring these in the search, which is correct
since they don't match the qualified relation name, and so there is no
real ambiguity.

Reported by Richard Guo.

Discussion: https://postgr.es/m/CAMbWs49MBjWYWDROJ8MZ%3DY%2B4UgRQa10wzik1tWrD5yto9eoGXg%40mail.gmail.com