Request that KNS cleans up its cache if there are dead entries

This requires (though not hard) the code found in
https://invent.kde.org/frameworks/knewstuff/-/merge_requests/23 and the
idea behind this is to clean up KNewStuff's knowledge of entries which
have been deleted without telling KNewStuff about it first. This is a
further step towards fixing the bug below.

CCBUG:417985

kcms/icons/icons.knsrc

@@ -21,5 +21,6 @@ ProvidersUrl=https://autoconfig.kde.org/ocs/providers.xml
 Categories=KDE Icon Theme
 TargetDir=icons
 Uncompress=true
+RemoveDeadEntries=true
 
 AdoptionCommand=@CMAKE_INSTALL_FULL_LIBEXECDIR@/plasma-changeicons %d