postgresql: Drop lowered max_wal_senders value.

This was originally added in a PostgreSQL 9.5 configuration, where the
default is 0, meaning replication is disabled[^1].  In PostgreSQL 10
and above, the default is 10[^2], and there is no reason to lower it
from that value.

[^1]: https://www.postgresql.org/docs/9.5/runtime-config-replication.html
[^2]: https://www.postgresql.org/docs/10/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-SENDER

puppet/zulip/templates/postgresql/12/postgresql.conf.template.erb

@@ -789,7 +789,6 @@ listen_addresses = <%= @listen_addresses %>
 
 <% if @s3_backups_bucket != '' -%>
 # Streaming backups and replication
-max_wal_senders = 5
 archive_mode = on
 archive_command = '/usr/bin/timeout 10m /usr/local/bin/env-wal-g wal-push %p'
 restore_command = '/usr/local/bin/env-wal-g wal-fetch "%f" "%p"'

puppet/zulip/templates/postgresql/13/postgresql.conf.template.erb

@@ -820,7 +820,6 @@ listen_addresses = <%= @listen_addresses %>
 
 <% if @s3_backups_bucket != '' -%>
 # Streaming backups and replication
-max_wal_senders = 5
 archive_mode = on
 archive_command = '/usr/bin/timeout 10m /usr/local/bin/env-wal-g wal-push %p'
 restore_command = '/usr/local/bin/env-wal-g wal-fetch "%f" "%p"'

puppet/zulip/templates/postgresql/14/postgresql.conf.template.erb

@@ -841,7 +841,6 @@ listen_addresses = <%= @listen_addresses %>
 
 <% if @s3_backups_bucket != '' -%>
 # Streaming backups and replication
-max_wal_senders = 5
 archive_mode = on
 archive_command = '/usr/bin/timeout 10m /usr/local/bin/env-wal-g wal-push %p'
 restore_command = '/usr/local/bin/env-wal-g wal-fetch "%f" "%p"'