Avoid sometimes printing both tables and their columns in DROP CASCADE.

Avoid sometimes printing both tables and their columns in DROP CASCADE.

A cascaded drop might find independent reasons to drop both a table
and some column of the table (for instance, a schema drop might include
dropping a data type used in some table in the schema).  Depending on
the order of visitation of pg_depend entries, we might report the
table column and the whole table as separate objects-to-be-dropped,
or we might only report the table.  This is confusing and leads to
unstable regression test output, so fix it to report only the table
regardless of visitation order.

Per gripe from Peter Geoghegan.  This is just cosmetic from a user's
standpoint, and we haven't actually seen regression test problems in
practice (yet), so I'll refrain from back-patching.

Discussion: https://postgr.es/m/15908.1547762076@sss.pgh.pa.us