Allow removal of Sub-series from parent Series

There is no way to disassociate a Sub-series from its parent Series. It's possible to accomplish the same goal by using workarounds, e.g. moving all Sub-series Titles to another Series and then deleting the empty Sub-series (when series deletion becomes available), but they are time-consuming. We need a new option made available on the Series page to "Remove Series from Parent Series". It should only be displayed when the current Series has a parent series.