Remove stable package repo for Ubuntu 20.04 from doc

[Dimitar Misev]

There are no stable packages for this OS yet, the docs need to be updated: ​https://doc.rasdaman.org/02_inst-guide.html#installation

[ocampos]

Edited text (waiting for revision):

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Uninstall rasdaman
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When uninstalling rasdaman, please execute the following commands to ensure
that all installed files and services are properly removed from the system.

.. hidden-code-block:: bash

    # Stop rasdaman
    $ service rasdaman stop
    # Disable rasdaman
    $ sudo systemctl disable rasdaman
    # Removes everything related to rasdaman (except dependencies)
    $ sudo apt purge rasdaman
    # Remove rasdaman folder
    $ sudo rm -r /opt/rasdaman
    # Remove rasdaman service
    $ sudo rm /etc/systemd/system/rasdaman.service
    # Remove rasdaman start/stop scripts
    $ sudo rm /etc/init.d/rasdaman
    # Remove rasdaman repository
    $ sudo rm /etc/apt/sources.list.d/rasdaman.list
    # Remove rasdaman startup file
    $ sudo rm /etc/profile.d/rasdaman.sh
    # Remove the rasdaman unit from systemctl --failed list
    $ sudo systemctl reset-failed
    # This will remove tomcat/postgres if no other package uses them.
    $ sudo apt-get autoremove

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Troubleshooting
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[ocampos]

Fixed and patched!