Remove unnecessary docs

docs/admin/03_psql.rst

-Installing AlekSIS with PostgreSQL backend
-==========================================
-
-PostgreSQL is the only supported database backend for AlekSIS. If you are
-installing AlekSIS manually, you need to properly set it up.
-
-Install the PostgreSQL server
------------------------------
-
-On Debian, install the postgresql server package with::
-
-  sudo apt install postgresql
-
-
-Create a database and user
---------------------------
-
-On Debian, you can use the following commands to create the database and a
-user who owns it::
-
-  sudo -u postgres createuser -D -P -R -S aleksis
-  sudo -u postgres createdb -E UTF-8 -O aleksis -T template0 -l C.UTF-8 aleksis
-
-When asked for the database user password, choose a secure, preferrably
-random, password. You can generate one using the pwgen utility if you like::
-
-  pwgen 16 1
-
-
-Configure AlekSIS to use PostgreSQL
------------------------------------
-
-Fill in the configuration under `/etc/aleksis/aleksis.toml` (or a file with any other name in this directory)::
-
-  [database]
-  host = "localhost"
-  name = "aleksis"
-  username = "aleksis"
-  password = "Y0urV3ryR4nd0mP4ssw0rd"
-
-Don't forget to run the migrations, like described in the basic setup guide.