pointless check in RelationBuildPartitionDesc

I noticed this strange hack in RelationBuildPartitionDesc:

/*
* It is possible that the pg_class tuple of a partition has not been
* updated yet to set its relpartbound field. The only case where
* this happens is when we open the parent relation to check using its
* partition descriptor that a new partition's bound does not overlap
* some existing partition.
*/
if (!((Form_pg_class) GETSTRUCT(tuple))->relispartition)
{
ReleaseSysCache(tuple);
continue;
}

After looking, it seems that this is just self-inflicted pain: for some
reason, we store the pg_inherits row for a partition, and immediately
afterwards compute and store its partition bound, which requires the
above hack. But if we do things in the opposite order, this is no
longer needed. I propose to remove it, as in the attached patch.

--
Álvaro Herrera Developer, https://www.PostgreSQL.org/