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