Disable refined preference scheme for implicits

Turns out it's not transitive when combined with the old scheme. So it
should be one or the other. Conservatively, this commit reverts to the old
scheme.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -920,7 +920,7 @@ trait Implicits { self: Typer =>
        *  `cand1` has been selected as an implicit more often than `cand2`.
        */
       def prefer(cand1: Candidate, cand2: Candidate): Boolean = {
-        val sym1 = cand1.ref.symbol
+        /*val sym1 = cand1.ref.symbol
         val sym2 = cand2.ref.symbol
         if (sym1.associatedFile == sym2.associatedFile) {
           val coord1 = sym1.coord
@@ -933,6 +933,7 @@ trait Implicits { self: Typer =>
           if (coord1.isIndex && coord2.isIndex)
             return coord1.toIndex < coord2.toIndex
         }
+        */
         ranking(cand1) < ranking(cand2)
       }