vacuumdb: Fix comment for vacuum_one_database().

vacuumdb: Fix comment for vacuum_one_database().

Since commit e0c2933a76, vacuum_one_database() always uses a
catalog query to discover the tables to process, but this comment
still notes the special case for which we used a catalog query
before that commit.  Let's just remove that note.

Also, commit 7781f4e3e7 renamed the "tables" parameter to "objects"
but missed updating this comment.  This commit fixes that as well.