Fix problem with treeview x resizing

The x size of the treeview plugin cannot really change expect if explicitly
resized.

The call to move_towards for x seems to raise a state where core.redraw is
always set to true and this prevent the application to go idle.

It is seen after the introduction of the dmon directory monitoring but it
is not clear why it wasn't seen before.

data/plugins/treeview.lua

@@ -238,8 +238,6 @@ function TreeView:update()
   if self.init_size then
     self.size.x = dest
     self.init_size = false
-  else
-    self:move_towards(self.size, "x", dest)
   end
 
   local duration = system.get_time() - self.tooltip.begin