Remove workaround from PostgreSQL replication docs

What does this MR do?

Removes a workaround from the PostgreSQL replication and failover docs which is not needed anymore (Fixed in omnibus-gitlab!6747 (merged)).

The change will be rendered here: https://docs.gitlab.com/ee/administration/postgresql/replication_and_failover.html#switch-the-application-to-use-the-new-cluster

Related issues & MRs

Relates omnibus-gitlab#7293 (closed)

Relates omnibus-gitlab#6430 (closed)

Depends on omnibus-gitlab!6747 (merged)

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