barLevel: Don't reset value on overview-start changes

While it makes sense to cap the current value to a new maximum value,
I can't think of a good reason why changing the overdrive-start should
reset the current value to that.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/654

js/ui/barLevel.js

@@ -89,7 +89,6 @@ var BarLevel = class {
                 `which is a number greater than the maximum allowed value ${this._maxValue}`);
 
         this._overdriveStart = value;
-        this._value = Math.max(Math.min(value, this._maxValue), 0);
         this.actor.queue_repaint();
     }