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/