Fix catalog data of pg_stop_backup(), labelled v2

Fix catalog data of pg_stop_backup(), labelled v2

This function has been incorrectly marked as a set-returning function
with prorows (estimated number of rows) set to 1 since its creation in
7117685, that introduced non-exclusive backups.  There is no need for
that as the function is designed to return only one tuple.

This commit fixes the catalog definition of pg_stop_backup_v2() so as it
is not marked as proretset anymore, with prorows set to 0.  This
simplifies its internals by removing one tuplestore (used for one single
record anyway) and by removing all the checks related to a set-returning
function.

Issue found during my quest to simplify some of the logic used in
in-core system functions.

Bump catalog version.

Reviewed-by: Aleksander Alekseev, Kyotaro Horiguchi
Discussion: https://postgr.es/m/Yh8guT78f1Ercfzw@paquier.xyz