[Finishes #74263536] start nectar no longer needed

Our simplification of Bee means that start nectar is no longer ever necessary.

I don't think I need any sort of migration for this since it's in the property bag.  Levelbuilder currently has one level with a start nectar defined (though said level is not part of any scripts right now).  I believe that levels that currently have start_nectar = "" will have the item removed from the property bag any time they're modified.  I could manually move it form the custom_levels.json but that seems unnecessary.