pgsql: Return ReplicationSlotAcquire API to its original form

Return ReplicationSlotAcquire API to its original form

Per 96540f80f833; the awkward API introduced by c6550776394e is no
longer needed.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20210408020913.zzprrlvqyvlt5cyy@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1632ea43682fcea8836ea245771ae85b9e1bcd38

Modified Files
--------------
src/backend/replication/logical/logicalfuncs.c |  2 +-
src/backend/replication/slot.c                 | 53 +++++++-------------------
src/backend/replication/slotfuncs.c            |  2 +-
src/backend/replication/walsender.c            |  4 +-
src/include/replication/slot.h                 | 10 +----
5 files changed, 18 insertions(+), 53 deletions(-)