Plasma 6 proposal: Remove Application Dashboard

Reasons why:

• It's technical architecture is questionable, and the way it's implemented complicates Kicker's code. Without it, a lot could be simplified which would make Kicker less buggy and let it use the default CompactRepresentation (see #85)
• Because of its questionable technical architecture, it's itself extremely buggy; see https://bugs.kde.org/buglist.cgi?component=Application%20Dashboard&resolution=---
• It's non-default, not officially maintained, and also not receiving any real community maintenance, unlike Kicker. Anecdotally, it's the least-used out of the 3 launchers we ship by default.
• As a result of those factors, it's currently completely broken in Plasma 6 (see https://bugs.kde.org/show_bug.cgi?id=470835) and nobody has stepped up to fix it.
• And anyway, there are already better full-screen dashboard launchers on store.kde.org, including one that's an improved and bugfixed fork of it (see https://store.kde.org/p/1897990).

Therefore I propose removing it from Plasma 6.