Simplify one use of ScanKey in pg_subscription.c

Simplify one use of ScanKey in pg_subscription.c

The section of the code in charge of returning all the relations
associated to a subscription only need one ScanKey, but allocated two of
them.  This code was introduced as a copy-paste from a different area on
the same file by 7c4f524, making the result confusing to follow.

Author: Peter Smith
Reviewed-by: Tom Lane, Julien Rouhaud, Bharath Rupireddy
Discussion: https://postgr.es/m/CAHut+PsLKe+rN3FjchoJsd76rx2aMsFTB7CTFxRgUP05p=kcpQ@mail.gmail.com