Remove unnecessary secondary index terms for replication settings

Hi,

In config.sgml, the entries for max_replication_slots and
max_active_replication_origins include secondary index terms:

<primary><varname>max_replication_slots</varname> configuration parameter</primary>
<secondary>in a sending server</secondary>

<primary><varname>max_active_replication_origins</varname> configuration parameter</primary>
<secondary>in a subscriber</secondary>

These secondary terms don't seem necessary anymore, since each parameter
now has only one index entry. Removing them would simplify the documentation.

Originally, the secondary entries made sense because
max_active_replication_origins was part of max_replication_slots,
so both needed separate index entries. But commit 04ff636cbce
split them into distinct parameters.

Patch attached.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION