migrate/default.xml.in: Remove unused plugin id
Fixes c1f3d925.
For some reason 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"