This situation with two clusters in Ubuntu may happen when upgrading to a newer release providing an newer postgresql version.
The automatic upgrade does not remove the old cluster, presumably for fear of erasing valuable data (which is wise because some postgres upgrades may require human work to be complete).
If you know you want to drop it, just run:
sudo pg_dropcluster --stop 9.1 main
The corresponding data directory will be removed and service postgresql
will no longer refer to 9.1
At this point the 9.2 cluster will still use the port 5433, which is unpractical.
To switch it to the default port, edit /etc/postgresql/9.2/main/postgresql.conf
and change the line port = 5433
to port = 5432
Then restart PostgreSQL.
Finally to get rid of the postgresql-9.1 packages see the result of dpkg -l 'postgresql*9.1*'