Consider removing postgresql-client

[StevenMaude]

In the discussion of #996:

#996 (comment)

there is a postgresql-client install within the Airlock Docker setup:

airlock/docker/dependencies.txt

Line 3 in 2f0c41a

postgresql-client

#996 (review)

I don't know why postgresql-client is installed, I suspect a c/p error - we don't use psql anywhere AFAICT