migrate/default.xml.in: Remove unused plugin id

Fixes c1f3d925.

For some reason (edit: ugh, so this is #871 (closed), starting from 46f43423 I guess) I am getting 'Plugin "(null)" could not be loaded, ...' dialog on a newly created VM (xfce4-panel 4.19.5). After clicking "Remove" I checked ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml and looks like this is the line that got "removed" 🤔