Only Empress should call drawTree()

[kwcantrell]

I'm not really a fan of having things call drawTree() from outside of the Empress object -- from the perspective of "managing the application state", I think it makes more sense to have Empress take care of that. I would suggest making empress.setTreeNodeVisibility() also call drawTree(), and adapting the second function here (for recentering) into another Empress function that centers the camera and calls drawTree().

(That said, I realize that there are 4 other things within the Side Panel Handler that call drawTree(), so ok if you'd prefer to keep this as is for the time being.)

Originally posted by @fedarko in https://github.com/biocore/empress/pull/212/files

[fedarko]

Worth noting that one of those "4 other things within the Side Panel Handler" is _updateLayout(), which as it turns out is unused (it's not referred to again within the code in favor of empress.updateLayout()). (Apparently I added this function a few months ago and then never used it again. My bad.)

... So we can probably just remove that function :)