Remove KDesktopFileActions::userDefinedServices

Most usages are broken due to KService internal changes. Also, this feature is only used by kdesvn and rabbitvcs (according to lxr and github search). Though for the latter the syntax is broken, because there are only two arguments provided. See https://github.com/blackkensai/rabbitvcs/blob/7b106a5e041f8bc7ca132de0e0b44c3bf6254908/clients/dolphin/rabbitvcs_svn_toplevel.desktop#L6C34-L6C55

This change is extraced from !1386 (6c604c33) in order to make the change sooner and not depend on the new KService/KConfig API that is currently in review.