[9.18] chg: doc: Remove -S changelog templates from open-source edition

These changelogs meant for -S edition were introduced to avoid rebase
conflicts. However, the same result can be achieved by linking the -S
changelogs directly from their open-source variants, rather than
including the -S changelogs directly in changelog.rst.

Mattermost discussion: https://mattermost.isc.org/isc/pl/1gioy1gatifttborqx5qhc9paw

Merge branch 'nicki/remove-s-specific-changelog-files-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10337