Fix ALTER SEQUENCE OWNED BY to not rewrite the sequence relation.

Fix ALTER SEQUENCE OWNED BY to not rewrite the sequence relation.

It's not necessary for it to do that, since OWNED BY requires only ordinary
catalog updates and doesn't affect future sequence values.  And pg_upgrade
needs to use OWNED BY without having it change the sequence's relfilenode.
Commit 3d79013b9 broke this by making all forms of ALTER SEQUENCE change
the relfilenode; that seems to be the explanation for the hard-to-reproduce
buildfarm failures we've been seeing since then.

Discussion: https://postgr.es/m/19785.1497215827@sss.pgh.pa.us