LP#2023555: remove references to Slony-I

LP#2023555: remove references to Slony-I

The Pg replication tool Slony-I is now obsolete for
new setups given the existence of mature native streaming
and logical replication support in PostgreSQL.

This patch removes an example script for setting up
Slony replication for Evergreen and edits a couple references
to Slony in source code comments.

No test plan: this patch is effectively a documentation patch.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>