LP#1882937: Comment and dead-code cleanup

LP#1882937: Comment and dead-code cleanup

Adjust comments in Storage::Driver::Pg to remove references to
Class::DBI::Replication, which we did not end up using.

Also remove a commented-out sub that purported to make use of a function
that did not exist in the home-grown multi-db implementation.

NOTE: While unused, the multi-db implementation in Storage::Driver::Pg
intelligently decides when to use a read-only secondary based on the
existence of a transaction or the "write-iness" state controlled by the
caller, and does not require separate main (or primary) and secondary
db lists.

Signed-off-by: Mike Rylander <mrylander@gmail.com>