Fuzzy thinking in is_publishable_class

is_publishable_class has a test "relid >= FirstNormalObjectId",
which I think we should drop, for two reasons:

1. It makes the comment claiming that this function tests the same
things as check_publication_add_relation a lie.

2. The comment about it claims that the purpose is to reject
information_schema relations, but if that's so, it's ineffective.
We consider it supported to drop and recreate information_schema,
and have indeed recommended doing so for some minor-version
upgrades. After that, the information_schema relations would no
longer have OIDs recognizable to this test.

So what is the motivation for this test? If there's an important
reason for it, we need to find a less fragile way to express it.

regards, tom lane