DEPR: remove PanelGroupBy, disable DataFrame.to_panel
My understanding is that we're removing Panel in 0.25.0.  A local attempt to do this all-at-once got messy quick (largely due to io.pytables and io.msgpack).  This gets the ball rolling by removing only PanelGroupBy and DataFrame.to_panel, followed by all of the code+tests that rely on either of these.