kcms/landingpage: remove KUserFeedback UI and code

The user is going to have a chance to turn on User Feedback in the new welcome wizard, which will be released with Plsma 5.27 and autostart for all users once. Thus, we no longer need to put this setting on the Quick Settings page to make sure the user sees it. This removes a lot of code and gives us space to add other settings in the future that might be more appropriate, like cursor speed or touchpad tap-to-click.

@mart @apol @teams/vdg

Here's how the page will look without it:


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