Removed TrackingConstraint

It's too hard to keep fixes up-to-date in both constraint implementations.
And anyway, OrderingConstraint is it for now.

Also added comment suggested by @smarter.

Author: odersky

src/dotty/tools/dotc/core/OrderingConstraint.scala

@@ -288,6 +288,10 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     newConstraint(boundsMap.updated(poly, entries1), lowerMap, upperMap).init(poly)
   }
 
+  /** Split dependent parameters off the bounds for parameters in `poly`.
+   *  Update all bounds to be normalized and update ordering to account for
+   *  dependent parameters.
+   */
   private def init(poly: PolyType)(implicit ctx: Context): This = {
     var current = this
     val loBuf, hiBuf = new mutable.ListBuffer[PolyParam]