Remove going-to-be-broken plugin "kde provider"
https://projects.kde.org/kde_projects.xml is removed in the next weeks as part of the reorganization happening in the move to gitlab by KDE.
Wanted: someone writing a replacement using gitlab, which then would also be usable for other gitlab servers.
@mwolff @apol I would do a 5.5.2 release upcoming Monday (May 25th) then, to get this out.