don't create storage when unnecessary

Some time ago, after partitioned indexes had been pushed, I realized
that even though I didn't want them to have relfilenodes, they did.  And
looking closer I noticed that *a lot* of relation kinds that didn't need
relfilenodes, had them anyway.

This patch fixes that; if no relfilenode is needed, it's not created.

I didn't verify pg_upgrade behavior across this commit.  Maybe something
needs tweaking there.


PS: I think it'd be worth following up with this ...
https://postgr.es/m/CAFjFpRcfzs+yst6YBCseD_orEcDNuAr9GUTraZ5GC=AvCYh55Q@mail.gmail.com

-- 
Álvaro Herrera