remove qt GUI, in preparation for new react gui in v1.0

related issue: #203 (closed)

This merge request removes the current qt gui, in preparation for the new react gui, as requested in issue #203 (closed)

Since we do not know much about the features of the new react gui, some relevant sections in the documentation now have a TODO notice, someone with more knowledge about the new GUI should revisit those.

changes:

• /gui/* has been deleted entirely
• the cmake scripts have been updated to remove the WITH_GUI switch
• sections of the documentation related to the qt gui have been removed
• sections of the documentation that may require updates for the new react gui have TODOs

Benjamin Hohl, on behalf of Mercedes-Benz Tech Innovation GmbH, Provider Information