Remove separator on the left-hand side of the clock

This separator was introduced to better distinguish the numbers of the
battery percentage and the clock. As the "power-manager" plugin was moved
left of the "pulseaudio" plugin, this separator isn't needed anymore.

The clock becomes "plugin-8" now. The plugin ID numbers of the lower panel
weren't changed to avoid upgrade issues with the launchers placed there,
as they additionally have local data stored in ~/.config/xfce4/panel.