Report a more useful error for reloptions on a partitioned table.

Previously, trying to set storage parameters on a partitioned table
always led to "unrecognized parameter foo", because the code expected
there might be some valid parameters; but there aren't any.  The docs
make clear that it's intended that there never will be any, so let's
replace this useless search with a more to-the-point message.

Simon Riggs and Karina Litskevich

Discussion: https://postgr.es/m/CANbhV-H=eZ9kTR9mUgKGK0Qv9uXP=U+dQg3rinQHfTdFMhBA2A@mail.gmail.com