Revert my ill-considered change that made formrdesc not insert the correct

relation rowtype OID into the relcache entries it builds.  This ensures
that catcache copies of the relation tupdescs will be fully correct.
While the deficiency doesn't seem to have any effect in the current
sources, we have been bitten by not-quite-right catcache tupdescs before,
so it seems like a good idea to maintain the rule that they should be right.