Do not cache BuddyMenu in favorites view

Let BuddyMenu be created on-demand.

Remove set_registered method, as this palette is now
created on-demand, makes no sense to modify the current
items as these will be set properly in the next
initialization.

Remove _palette_enabled as this is deprecated code
from previous palette implementation.

Remove self._register_menu and simplify code.

This partch avoids #2184 issue for the buddy icon
palette in favorites view.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>