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