doc: remove unnecessary secondary index terms for replication settings.

doc: remove unnecessary secondary index terms for replication settings.

Previously, config.sgml included secondary index terms for
max_replication_slots and max_active_replication_origins. These are
no longer necessary, as each parameter now has a single distinct index entry.

The secondary index terms were originally useful because
max_active_replication_origins was part of max_replication_slots,
and separate index entries helped users locate each setting. However,
commit 04ff636cbce split them into independent parameters,
making the secondary terms redundant.

This commit removes the unnecessary secondary index entries to
simplify the documentation.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://postgr.es/m/e825e7a7-4877-441d-93c1-25377db36c31@oss.nttdata.com